manifest: support optional --manifest-name

Still use the repo manifest by default as before, but gives us
the option of overriding it to support e.g.: using a subset of
the full manifest.

Change-Id: Ia42cd1cb3a0a58929d31bb01c9724e9d31f68730
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256372
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Sean McAllister <smcallis@google.com>