allow -t abbreviation for --track in git branch

also makes it consistent with git-checkout
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-branch.txt

@@ -111,6 +111,7 @@ OPTIONS
 --no-abbrev::
 	Display the full sha1s in the output listing rather than abbreviating them.
 
+-t::
 --track::
 	When creating a new branch, set up configuration to mark the
 	start-point branch as "upstream" from the new branch. This

builtin-branch.c

@@ -547,7 +547,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	struct option options[] = {
 		OPT_GROUP("Generic options"),
 		OPT__VERBOSE(&verbose),
-		OPT_SET_INT( 0 , "track",  &track, "set up tracking mode (see git-pull(1))",
+		OPT_SET_INT('t', "track",  &track, "set up tracking mode (see git-pull(1))",
 			BRANCH_TRACK_EXPLICIT),
 		OPT_BOOLEAN( 0 , "color",  &branch_use_color, "use colored output"),
 		OPT_SET_INT('r', NULL,     &kinds, "act on remote-tracking branches",