Improve how "likely an AOSP change" is detected

git branch -vv can be incredibly slow (over 5s on my machine/repo), so
when aosp_sha.sh is called multiple times per commit, the repo upload
hook can take a very long time to run (>2 mins for my stack of changes).

The only output used from git branch -vv was the upstream of the current
branch, which can be determined in a much faster/simpler way via
printing the ref name of @{u} (git lingo for the upstream HEAD).

Test: $ { aosp_sha.sh HEAD && echo ok || echo no } in aosp and internal
Change-Id: I993496d8ec9e55c13803590bbb6c5d9d49cde158

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