init: rename --repo-branch to --repo-rev

We refer to this as "revision" in help text, and in REPO_REV envvar,
so rename to --repo-rev to be consistent.  We keep --repo-branch for
backwards compatibility, but as a hidden option.

Bug: https://crbug.com/gerrit/11045
Change-Id: I1ecc282fba32917ed78a63850360c08469db849a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/259352
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>