t7610: test for mktemp before test execution

mktemp is not available on all platforms, so the test
'temporary filenames are used with mergetool.writeToTemp'
fails there.
This patch does not replace mktemp but just disables
the test that otherwise would fail.
mergetool checks itself before executing mktemp and
reports an error.

Signed-off-by: Armin Kunaschik <megabreit@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7610-mergetool.sh

@@ -589,7 +589,12 @@ test_expect_success 'filenames seen by tools start with ./' '
 	git reset --hard master >/dev/null 2>&1
 '
 
-test_expect_success 'temporary filenames are used with mergetool.writeToTemp' '
+test_lazy_prereq MKTEMP '
+	tempdir=$(mktemp -d -t foo.XXXXXX) &&
+	test -d "$tempdir"
+'
+
+test_expect_success MKTEMP 'temporary filenames are used with mergetool.writeToTemp' '
 	git checkout -b test16 branch1 &&
 	test_config mergetool.writeToTemp true &&
 	test_config mergetool.myecho.cmd "echo \"\$LOCAL\"" &&