Revert "git_cache: Remove locks"

This reverts commit c3eb3fa3.

Reason for revert: Broke everything, probably due to version pin mismatch somewhere (gclient/git_cache likely getting called across repo boundary)

Original change's description:
> git_cache: Remove locks
> 
> These aren't in use, and the original problem they were
> meant to solve has been solved at the gclient.py layer 
> using resource locking:
>   https://codereview.chromium.org/2049583003
> 
> Bug: 773008
> Change-Id: I6609f39d7f15604e0bb3d742a41c4f9fec87a57a
> Reviewed-on: https://chromium-review.googlesource.com/707728
> Reviewed-by: Aaron Gable <agable@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Ryan Tseng <hinoka@chromium.org>

TBR=iannucci@chromium.org,hinoka@chromium.org,agable@chromium.org,phajdan.jr@chromium.org

Change-Id: Ia9061f0ac827599d81d7638f535f97e15efa75fe
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 773008
Reviewed-on: https://chromium-review.googlesource.com/711055Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>