In theory this should prevent `git rebase-update` from invoking
potentially expensive `git gc` operations after every `git rebase`
(ideally once per branch, but sometimes as many as three per branch (!)).

This has outsized effects as the size of the repo or number of branches
increases, and additionally has penalties on Windows due to git's
POSIXey assumptions around how disk I/O works.

After doing all rebase stuff, we run `git gc --auto` from rebase-update
once (assuming we were successful; if the command failed then we don't
run gc at all).

R=apolito@google.com

Bug: 617218
Change-Id: I37a8f7953779f15b320e4ed7bb85119cb4bb665a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2868766
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>