shortlog: match commit trailers with --group

If a project uses commit trailers, this patch lets you use
shortlog to see who is performing each action. For example,
running:

  git shortlog -ns --group=trailer:reviewed-by

in git.git shows who has reviewed. You can even use a custom
format to see things like who has helped whom:

  git shortlog --format="...helped %an (%ad)" \
               --group=trailer:helped-by

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/Documentation/git-shortlog.txt b/Documentation/git-shortlog.txt
index 6496d31..edd6cda 100644
--- a/Documentation/git-shortlog.txt
+++ b/Documentation/git-shortlog.txt
@@ -53,6 +53,19 @@ OPTIONS
 +
  - `author`, commits are grouped by author
  - `committer`, commits are grouped by committer (the same as `-c`)
+ - `trailer:<field>`, the `<field>` is interpreted as a case-insensitive
+   commit message trailer (see linkgit:git-interpret-trailers[1]). For
+   example, if your project uses `Reviewed-by` trailers, you might want
+   to see who has been reviewing with
+   `git shortlog -ns --group=trailer:reviewed-by`.
++
+Note that commits that do not include the trailer will not be counted.
+Likewise, commits with multiple trailers (e.g., multiple signoffs) may
+be counted more than once.
++
+The contents of each trailer value are taken literally and completely.
+No mailmap is applied, and the `-e` option has no effect (if the trailer
+contains a username and email, they are both always shown).
 
 -c::
 --committer::