Instead of creating an empty dict and setting all the required git variables
there, add those variables to os.environ so that we do not drop other
required variables -- in my case, git was not in /usr/bin so invoking 'git
commit' in _add_schema_commit fails with "no such file or directory".

Change-Id: I8c9b14e1f534efc8f40d21b312a5396b2f65f4a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1528290
Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>