Better usage string for reflog.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-reflog.c

diff --git a/builtin-reflog.c b/builtin-reflog.c
index ddfdf5a..95198c5 100644
--- a/builtin-reflog.c
+++ b/builtin-reflog.c
@@ -694,7 +694,7 @@
  */
 
 static const char reflog_usage[] =
-"git reflog (expire | ...)";
+"git reflog [ show | expire | delete ]";
 
 int cmd_reflog(int argc, const char **argv, const char *prefix)
 {