git-am: fix shell quoting

Noticed by Stephan Beyer; the new test is mine.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4252-am-options.sh

@@ -58,4 +58,12 @@ test_expect_success 'interrupted am --directory="frotz nitfol"' '
 	grep One "frotz nitfol/file-5"
 '
 
+test_expect_success 'apply to a funny path' '
+	with_sq="with'\''sq"
+	rm -fr .git/rebase-apply &&
+	git reset --hard initial &&
+	git am --directory="$with_sq" "$tm"/am-test-5-2 &&
+	test -f "$with_sq/file-5"
+'
+
 test_done