project: Skip superproject upstream check for MetaProjects

The superproject-gated upstream check in _CheckForImmutableRevision
only applies to user projects listed in the manifest. MetaProjects
(ManifestProject and RepoProject) never participate in a superproject
relationship, so evaluating git_superproject.UseSuperproject(...,
self.manifest) for them serves no purpose and, worse, calls the
manifest.superproject property which forces a manifest load.

During repo init, ManifestProject._ConfigureDependencies calls
self.Sync_NetworkHalf before manifest.xml has been linked into
.repo/. That reaches _CheckForImmutableRevision, which triggered the
manifest load and failed with:

  ManifestParseError: .../.repo/manifest.xml: [Errno 2] No such file
  or directory

breaking fresh repo init with SHA-based --manifest-branch combined
with --manifest-upstream-branch.

Factor the "should we consult the superproject for upstream?"
decision into a small overridable hook, _UseSuperprojectForUpstream.
Project's default delegates to git_superproject.UseSuperproject;
MetaProject overrides it to return False, localizing the
MetaProject-specific behavior to MetaProject.

Test verifies that calling _CheckForImmutableRevision on a
ManifestProject whose manifest.xml is not yet on disk returns False
without raising and does not create the file.

Change-Id: I22059109243d914036c06c6fe0081a5aba05da89
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/574201
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Ajay Gupta <ajagup@qti.qualcomm.com>
Tested-by: Ajay Gupta <ajagup@qti.qualcomm.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Nasser Grainawi <nasser.grainawi@oss.qualcomm.com>