GIT_TRACE_PERFORMANCE shows invocation time of each subcommands.
https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables

This is tested on
https://ci.chromium.org/swarming/task/3d605b8aab1a5a10?server=chromium-swarm.appspot.com
bot_update ran successfully.

Bug: 840592
Change-Id: I65ef733d1473f9f0f7e0109b0f727595766021b6
Reviewed-on: https://chromium-review.googlesource.com/1053028
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>