Don't clobber the PATH environment variable when running git commit.

Don't clobber the PATH environment variable when running git commit.

OS X comes with an old version of Python, so we install a newer one in
/usr/local/git/bin and add that to the PATH in the build step.  The old version
that comes with OS X doesn't understand the timezone in GIT_*_DATE environment
variables, so always creates commits in the local timezone (US/Pacific).  This
leads to commits with different hashes to what the test was expecting, causing
failures on OS X 10.6 and 10.7:

http://build.chromium.org/p/chromium.infra/builders/infra-continuous-mac-10.6-64/builds/3/steps/infra%20python%20tests/logs/stdio

BUG=
R=vadimsh@chromium.org

Committed: https://chromium.googlesource.com/infra/testing/testing_support/+/3bb4723ba5de249411172b63de6112461a8278d1

[dsansome, 2015-09-08 07:23:08]

dsansome@chromium.org changed reviewers:
+ pgervais@chromium.org

[dsansome, 2015-09-08 22:35:12]

dsansome@chromium.org changed reviewers:
+ vadimsh@chromium.org
- pgervais@chromium.org

[dsansome, 2015-09-08 22:35:12]

Oops, pgervais is OOO.  Could you take a look at this instead?

[Vadim Sh., 2015-09-08 22:41:01]

lgtm

[dsansome, 2015-09-08 22:43:52]

Committed patchset #1 (id:1) manually as
3bb4723ba5de249411172b63de6112461a8278d1 (presubmit successful).