perf: accommodate for MacOSX

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>