Add a performance test for git-grep

The only catch is that we don't really know what our repo contains, so
we have to ignore any possible "not found" status from git-grep.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/p7810-grep.sh

@@ -0,0 +1,23 @@
+#!/bin/sh
+
+test_description="git-grep performance in various modes"
+
+. ./perf-lib.sh
+
+test_perf_large_repo
+test_checkout_worktree
+
+test_perf 'grep worktree, cheap regex' '
+	git grep some_nonexistent_string || :
+'
+test_perf 'grep worktree, expensive regex' '
+	git grep "^.* *some_nonexistent_string$" || :
+'
+test_perf 'grep --cached, cheap regex' '
+	git grep --cached some_nonexistent_string || :
+'
+test_perf 'grep --cached, expensive regex' '
+	git grep --cached "^.* *some_nonexistent_string$" || :
+'
+
+test_done