[PATCH v3 2/3] patch-id: document new behaviour

["Michael S. Tsirkin" <mst@redhat.com>]

Clarify that patch ID is now a sum of hashes, not a hash.
Document --stable and --unstable flags.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

changes from v2:
	explicitly list the kinds of changes against which patch ID is stable

 Documentation/git-patch-id.txt | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-patch-id.txt b/Documentation/git-patch-id.txt
index 312c3b1..30923e0 100644
--- a/Documentation/git-patch-id.txt
+++ b/Documentation/git-patch-id.txt
@@ -8,14 +8,14 @@ git-patch-id - Compute unique ID for a patch
 SYNOPSIS
 --------
 [verse]
-'git patch-id' < <patch>
+'git patch-id' [--stable | --unstable] < <patch>
 
 DESCRIPTION
 -----------
-A "patch ID" is nothing but a SHA-1 of the diff 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.
+A "patch ID" is nothing but a sum of SHA-1 of the diff hunks 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.
 
@@ -27,6 +27,19 @@ This can be used to make a mapping from patch ID to commit ID.
 
 OPTIONS
 -------
+
+--stable::
+	Use a symmetrical sum of hashes as the patch ID.
+	With this option, reordering file diffs that make up a patch or
+	splitting a diff up to multiple diffs that touch the same path
+	does not affect the ID.
+	This is the default.
+
+--unstable::
+	Use a non-symmetrical sum of hashes, such that reordering
+	or splitting the patch does affect the ID.
+	This was the default value for git 1.9 and older.
+
 <patch>::
 	The diff to create the ID of.
 
-- 
MST