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, 2 insertions, 0 deletions

diff --git a/shortlog.h b/shortlog.h
index 876a521..89c2dbc 100644
--- a/shortlog.h
+++ b/shortlog.h
@@ -19,7 +19,9 @@ struct shortlog {
 	enum {
 		SHORTLOG_GROUP_AUTHOR = 0,
 		SHORTLOG_GROUP_COMMITTER,
+		SHORTLOG_GROUP_TRAILER,
 	} group;
+	char *trailer;
 
 	char *common_repo_prefix;
 	int email;