Add the support to check how far a project is behind upstream git.

Still need to manually run git merge. But this tool can help update
METADATA. :)

Test: ./updater.sh update perf_data_converter
Change-Id: Ic46a0eb723ae22f0fc7d61a67a14299761b564a4

Bug: 109748616
Test: https://android-review.googlesource.com/c/platform/external/kotlinc/+/699886
Change-Id: I1c28aa256bca6ee5be1ea15f295c5e0fa63526d1