Update _CheckForImmutableRevision to use git rev-list

_CheckForImmutableRevision is used to see if repo can
skip fetching a project, but 'git rev-parse' with partial
clone does a data fetch to accomplish this.

Changed to use: 'git rev-list -1 --missing=allow-any <SHA>^0' which
checks the local ref without fetching from the server first.

Bug: [google internal] b/179477822

Testing:
- Unit tests
- Verified init/sync working on aosp-master
- Verified wwith a pinned manifest that local ref check works (no fetch)

Change-Id: If327b893c6658421f41df1f58c337f53b4c60ce6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296142


Reviewed-by: Dan Willemsen <dwillemsen@google.com>
Tested-by: Ian Kasprzak <iankaz@google.com>