git-grep: fix 'git grep -e $pattern' handling

People typically say 'grep -e $pattern' because $pattern has a leading
dash which would be mistaken as a grep flag.  Make sure we pass -e in
front of $pattern when we invoke grep.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-grep.sh

@@ -40,4 +40,4 @@ while : ; do
 	shift
 done
 git-ls-files -z "${git_flags[@]}" "$@" |
-	xargs -0 grep "${flags[@]}" "$pattern"
+	xargs -0 grep "${flags[@]}" -e "$pattern"