object name: introduce ':/<oneline prefix>' notation

To name a commit, you can now say

	$ git rev-parse ':/Initial revision of "git"'

and it will return the hash of the youngest commit whose
commit message (the oneline) begins with the given prefix.

For future extension, a leading exclamation mark is treated
specially: if you want to match a commit message starting with
a '!', just repeat the exclamation mark. So, to match a commit
which starts with '!Hello World', use

	$ git show ':/!!Hello World'

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/git-rev-parse.txt b/Documentation/git-rev-parse.txt
index 4041a16070..ccc66aae7f 100644
--- a/Documentation/git-rev-parse.txt
+++ b/Documentation/git-rev-parse.txt
@@ -190,6 +190,13 @@ blobs contained in a commit.
   and dereference the tag recursively until a non-tag object is
   found.
 
+* A colon, followed by a slash, followed by a text: this names
+  a commit whose commit message starts with the specified text.
+  This name returns the youngest matching commit which is
+  reachable from any ref.  If the commit message starts with a
+  '!', you have to repeat that;  the special sequence ':/!',
+  followed by something else than '!' is reserved for now.
+
 * A suffix ':' followed by a path; this names the blob or tree
   at the given path in the tree-ish object named by the part
   before the colon.