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diff --git a/crates/cargo-test-support/src/git.rs b/crates/cargo-test-support/src/git.rs
new file mode 100644
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+++ b/crates/cargo-test-support/src/git.rs
@@ -0,0 +1,249 @@
+/*
+# Git Testing Support
+
+## Creating a git dependency
+`git::new()` is an easy way to create a new git repository containing a
+project that you can then use as a dependency. It will automatically add all
+the files you specify in the project and commit them to the repository.
+Example:
+
+```
+let git_project = git::new("dep1", |project| {
+    project
+        .file("Cargo.toml", &basic_manifest("dep1"))
+        .file("src/lib.rs", r#"pub fn f() { println!("hi!"); } "#)
+});
+
+// Use the `url()` method to get the file url to the new repository.
+let p = project()
+    .file("Cargo.toml", &format!(r#"
+        [package]
+        name = "a"
+        version = "1.0.0"
+
+        [dependencies]
+        dep1 = {{ git = '{}' }}
+    "#, git_project.url()))
+    .file("src/lib.rs", "extern crate dep1;")
+    .build();
+```
+
+## Manually creating repositories
+`git::repo()` can be used to create a `RepoBuilder` which provides a way of
+adding files to a blank repository and committing them.
+
+If you want to then manipulate the repository (such as adding new files or
+tags), you can use `git2::Repository::open()` to open the repository and then
+use some of the helper functions in this file to interact with the repository.
+
+*/
+
+use crate::{path2url, project, Project, ProjectBuilder};
+use std::fs;
+use std::path::{Path, PathBuf};
+use std::sync::Once;
+use url::Url;
+
+#[must_use]
+pub struct RepoBuilder {
+    repo: git2::Repository,
+    files: Vec<PathBuf>,
+}
+
+pub struct Repository(git2::Repository);
+
+/// Create a `RepoBuilder` to build a new git repository.
+///
+/// Call `build()` to finalize and create the repository.
+pub fn repo(p: &Path) -> RepoBuilder {
+    RepoBuilder::init(p)
+}
+
+impl RepoBuilder {
+    pub fn init(p: &Path) -> RepoBuilder {
+        t!(fs::create_dir_all(p.parent().unwrap()));
+        let repo = init(p);
+        RepoBuilder {
+            repo,
+            files: Vec::new(),
+        }
+    }
+
+    /// Add a file to the repository.
+    pub fn file(self, path: &str, contents: &str) -> RepoBuilder {
+        let mut me = self.nocommit_file(path, contents);
+        me.files.push(PathBuf::from(path));
+        me
+    }
+
+    /// Add a file that will be left in the working directory, but not added
+    /// to the repository.
+    pub fn nocommit_file(self, path: &str, contents: &str) -> RepoBuilder {
+        let dst = self.repo.workdir().unwrap().join(path);
+        t!(fs::create_dir_all(dst.parent().unwrap()));
+        t!(fs::write(&dst, contents));
+        self
+    }
+
+    /// Create the repository and commit the new files.
+    pub fn build(self) -> Repository {
+        {
+            let mut index = t!(self.repo.index());
+            for file in self.files.iter() {
+                t!(index.add_path(file));
+            }
+            t!(index.write());
+            let id = t!(index.write_tree());
+            let tree = t!(self.repo.find_tree(id));
+            let sig = t!(self.repo.signature());
+            t!(self
+                .repo
+                .commit(Some("HEAD"), &sig, &sig, "Initial commit", &tree, &[]));
+        }
+        let RepoBuilder { repo, .. } = self;
+        Repository(repo)
+    }
+}
+
+impl Repository {
+    pub fn root(&self) -> &Path {
+        self.0.workdir().unwrap()
+    }
+
+    pub fn url(&self) -> Url {
+        path2url(self.0.workdir().unwrap().to_path_buf())
+    }
+
+    pub fn revparse_head(&self) -> String {
+        self.0
+            .revparse_single("HEAD")
+            .expect("revparse HEAD")
+            .id()
+            .to_string()
+    }
+}
+
+/// Initialize a new repository at the given path.
+pub fn init(path: &Path) -> git2::Repository {
+    default_search_path();
+    let repo = t!(git2::Repository::init(path));
+    default_repo_cfg(&repo);
+    repo
+}
+
+fn default_search_path() {
+    use crate::paths::global_root;
+    use git2::{opts::set_search_path, ConfigLevel};
+
+    static INIT: Once = Once::new();
+    INIT.call_once(|| unsafe {
+        let path = global_root().join("blank_git_search_path");
+        t!(set_search_path(ConfigLevel::System, &path));
+        t!(set_search_path(ConfigLevel::Global, &path));
+        t!(set_search_path(ConfigLevel::XDG, &path));
+        t!(set_search_path(ConfigLevel::ProgramData, &path));
+    })
+}
+
+fn default_repo_cfg(repo: &git2::Repository) {
+    let mut cfg = t!(repo.config());
+    t!(cfg.set_str("user.email", "foo@bar.com"));
+    t!(cfg.set_str("user.name", "Foo Bar"));
+}
+
+/// Create a new git repository with a project.
+pub fn new<F>(name: &str, callback: F) -> Project
+where
+    F: FnOnce(ProjectBuilder) -> ProjectBuilder,
+{
+    new_repo(name, callback).0
+}
+
+/// Create a new git repository with a project.
+/// Returns both the Project and the git Repository.
+pub fn new_repo<F>(name: &str, callback: F) -> (Project, git2::Repository)
+where
+    F: FnOnce(ProjectBuilder) -> ProjectBuilder,
+{
+    let mut git_project = project().at(name);
+    git_project = callback(git_project);
+    let git_project = git_project.build();
+
+    let repo = init(&git_project.root());
+    add(&repo);
+    commit(&repo);
+    (git_project, repo)
+}
+
+/// Add all files in the working directory to the git index.
+pub fn add(repo: &git2::Repository) {
+    // FIXME(libgit2/libgit2#2514): apparently, `add_all` will add all submodules
+    // as well, and then fail because they're directories. As a stop-gap, we just
+    // ignore all submodules.
+    let mut s = t!(repo.submodules());
+    for submodule in s.iter_mut() {
+        t!(submodule.add_to_index(false));
+    }
+    let mut index = t!(repo.index());
+    t!(index.add_all(
+        ["*"].iter(),
+        git2::IndexAddOption::DEFAULT,
+        Some(
+            &mut (|a, _b| if s.iter().any(|s| a.starts_with(s.path())) {
+                1
+            } else {
+                0
+            })
+        )
+    ));
+    t!(index.write());
+}
+
+/// Add a git submodule to the repository.
+pub fn add_submodule<'a>(
+    repo: &'a git2::Repository,
+    url: &str,
+    path: &Path,
+) -> git2::Submodule<'a> {
+    let path = path.to_str().unwrap().replace(r"\", "/");
+    let mut s = t!(repo.submodule(url, Path::new(&path), false));
+    let subrepo = t!(s.open());
+    default_repo_cfg(&subrepo);
+    t!(subrepo.remote_add_fetch("origin", "refs/heads/*:refs/heads/*"));
+    let mut origin = t!(subrepo.find_remote("origin"));
+    t!(origin.fetch(&Vec::<String>::new(), None, None));
+    t!(subrepo.checkout_head(None));
+    t!(s.add_finalize());
+    s
+}
+
+/// Commit changes to the git repository.
+pub fn commit(repo: &git2::Repository) -> git2::Oid {
+    let tree_id = t!(t!(repo.index()).write_tree());
+    let sig = t!(repo.signature());
+    let mut parents = Vec::new();
+    if let Some(parent) = repo.head().ok().map(|h| h.target().unwrap()) {
+        parents.push(t!(repo.find_commit(parent)))
+    }
+    let parents = parents.iter().collect::<Vec<_>>();
+    t!(repo.commit(
+        Some("HEAD"),
+        &sig,
+        &sig,
+        "test",
+        &t!(repo.find_tree(tree_id)),
+        &parents
+    ))
+}
+
+/// Create a new tag in the git repository.
+pub fn tag(repo: &git2::Repository, name: &str) {
+    let head = repo.head().unwrap().target().unwrap();
+    t!(repo.tag(
+        name,
+        &t!(repo.find_object(head, None)),
+        &t!(repo.signature()),
+        "make a new tag",
+        false
+    ));
+}