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+git-patch-id(1)
+===============
+
+NAME
+----
+git-patch-id - Compute unique ID for a patch
+
+SYNOPSIS
+--------
+[verse]
+'git patch-id' [--stable | --unstable]
+
+DESCRIPTION
+-----------
+Read a patch from the standard input and compute the patch ID for it.
+
+A "patch ID" is nothing but a sum of SHA-1 of the file diffs associated with a
+patch, with whitespace and line numbers ignored.  As such, it's "reasonably
+stable", but at the same time also reasonably unique, i.e., two patches that
+have the same "patch ID" are almost guaranteed to be the same thing.
+
+IOW, you can use this thing to look for likely duplicate commits.
+
+When dealing with 'git diff-tree' output, it takes advantage of
+the fact that the patch is prefixed with the object name of the
+commit, and outputs two 40-byte hexadecimal strings.  The first
+string is the patch ID, and the second string is the commit ID.
+This can be used to make a mapping from patch ID to commit ID.
+
+OPTIONS
+-------
+
+--stable::
+	Use a "stable" sum of hashes as the patch ID. With this option:
+	 - Reordering file diffs that make up a patch does not affect the ID.
+	   In particular, two patches produced by comparing the same two trees
+	   with two different settings for "-O<orderfile>" result in the same
+	   patch ID signature, thereby allowing the computed result to be used
+	   as a key to index some meta-information about the change between
+	   the two trees;
+
+	 - Result is different from the value produced by git 1.9 and older
+	   or produced when an "unstable" hash (see --unstable below) is
+	   configured - even when used on a diff output taken without any use
+	   of "-O<orderfile>", thereby making existing databases storing such
+	   "unstable" or historical patch-ids unusable.
+
+	This is the default if patchid.stable is set to true.
+
+--unstable::
+	Use an "unstable" hash as the patch ID. With this option,
+	the result produced is compatible with the patch-id value produced
+	by git 1.9 and older.  Users with pre-existing databases storing
+	patch-ids produced by git 1.9 and older (who do not deal with reordered
+	patches) may want to use this option.
+
+	This is the default.
+
+GIT
+---
+Part of the linkgit:git[1] suite