Bail out as soon as something goes wrong. Previously if we failed early,
we'd obliterate the existing version anyway.

Don't remove the temporary directory on failure. Otherwise there's no
way to debug a failure.

Bug: N/A
Test: updated external/curl
Change-Id: I3bf055a8ec514c7b76683baee8e1540deb5a3e5d

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