perf: accommodate for MacOSX

As this developer has no access to MacOSX developer setups anymore,
Travis becomes the best bet to run performance tests on that OS.

However, on MacOSX /usr/bin/time is that good old BSD executable that
no Linux user cares about, as demonstrated by the perf-lib.sh's use
of GNU-ish extensions. And by the hard-coded path.

Let's just work around this issue by using gtime on MacOSX, the
Homebrew-provided GNU implementation onto which pretty much every
MacOSX power user falls back anyway.

To help other developers use Travis to run performance tests on
MacOSX, the .travis.yml file now sports a commented-out line that
installs GNU time via Homebrew.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.travis.yml

@@ -82,6 +82,8 @@ before_install:
       brew tap homebrew/binary --quiet
       brew_force_set_latest_binary_hash perforce
       brew_force_set_latest_binary_hash perforce-server
+      # Uncomment this if you want to run perf tests:
+      # brew install gnu-time
       brew install git-lfs perforce-server perforce gettext
       brew link --force gettext
       ;;

t/perf/perf-lib.sh

@@ -127,11 +127,15 @@ test_checkout_worktree () {
 # Performance tests should never fail.  If they do, stop immediately
 immediate=t
 
+# Perf tests require GNU time
+case "$(uname -s)" in Darwin) GTIME="${GTIME:-gtime}";; esac
+GTIME="${GTIME:-/usr/bin/time}"
+
 test_run_perf_ () {
 	test_cleanup=:
 	test_export_="test_cleanup"
 	export test_cleanup test_export_
-	/usr/bin/time -f "%E %U %S" -o test_time.$i "$SHELL" -c '
+	"$GTIME" -f "%E %U %S" -o test_time.$i "$SHELL" -c '
 . '"$TEST_DIRECTORY"/test-lib-functions.sh'
 test_export () {
 	[ $# != 0 ] || return 0