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+Shallow commits
+===============
+
+.Definition
+*********************************************************
+Shallow commits do have parents, but not in the shallow
+repo, and therefore grafts are introduced pretending that
+these commits have no parents.
+*********************************************************
+
+$GIT_DIR/shallow lists commit object names and tells Git to
+pretend as if they are root commits (e.g. "git log" traversal
+stops after showing them; "git fsck" does not complain saying
+the commits listed on their "parent" lines do not exist).
+
+Each line contains exactly one object name. When read, a commit_graft
+will be constructed, which has nr_parent < 0 to make it easier
+to discern from user provided grafts.
+
+Note that the shallow feature could not be changed easily to
+use replace refs: a commit containing a `mergetag` is not allowed
+to be replaced, not even by a root commit. Such a commit can be
+made shallow, though. Also, having a `shallow` file explicitly
+listing all the commits made shallow makes it a *lot* easier to
+do shallow-specific things such as to deepen the history.
+
+Since fsck-objects relies on the library to read the objects,
+it honours shallow commits automatically.
+
+There are some unfinished ends of the whole shallow business:
+
+- maybe we have to force non-thin packs when fetching into a
+  shallow repo (ATM they are forced non-thin).
+
+- A special handling of a shallow upstream is needed. At some
+  stage, upload-pack has to check if it sends a shallow commit,
+  and it should send that information early (or fail, if the
+  client does not support shallow repositories). There is no
+  support at all for this in this patch series.
+
+- Instead of locking $GIT_DIR/shallow at the start, just
+  the timestamp of it is noted, and when it comes to writing it,
+  a check is performed if the mtime is still the same, dying if
+  it is not.
+
+- It is unclear how "push into/from a shallow repo" should behave.
+
+- If you deepen a history, you'd want to get the tags of the
+  newly stored (but older!) commits. This does not work right now.
+
+To make a shallow clone, you can call "git-clone --depth 20 repo".
+The result contains only commit chains with a length of at most 20.
+It also writes an appropriate $GIT_DIR/shallow.
+
+You can deepen a shallow repository with "git-fetch --depth 20
+repo branch", which will fetch branch from repo, but stop at depth
+20, updating $GIT_DIR/shallow.
+
+The special depth 2147483647 (or 0x7fffffff, the largest positive
+number a signed 32-bit integer can contain) means infinite depth.