Cache built Bazel binaries by commit sha.

[tjgq]

The commit to build Bazel at can be given as a branch or tag name, which may point to different commits over time. This could cause a previously built Bazel binary to be incorrectly reused.

[joeleba]

The commit to build Bazel at can be given as a branch or tag name

From reading the documentation in the code, it certainly wasn't the intended usage. commit was meant for commits only. Same for the --bazel_commits flag.

If you have a use case for using branch or tag name there, I think the proper thing to do is renaming the --bazel_commits flag to --bazel_identifier--bazel_from_source_at, then do what you're doing in this PR (getting the actual commit for caching the binary). That'd allow us to not overload the term "commit".

[joeleba]

In that case, should this parameter be changed to version_identifier or something, instead of commit?