shortlog: mention the "-e" option in the usage

Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-shortlog.c

@@ -8,7 +8,7 @@
 #include "mailmap.h"
 
 static const char shortlog_usage[] =
-"git-shortlog [-n] [-s] [<commit-id>... ]";
+"git-shortlog [-n] [-s] [-e] [<commit-id>... ]";
 
 static char *common_repo_prefix;
 static int email;