git-grep: fix exit code when we use external grep.
git-grep: fix exit code when we use external grep.

Upon hit, we should exit with status 0.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git