project: set core.repositoryFormatVersion=1 when using extensions

When using extensions, make sure we set the git repo format version
so git knows to check the extension compatibility.  We can add a
helper to the Project API to simplify this and make it foolproof.

Change-Id: I9ab6c32d92fe2b8e5df6e2b080ca71556332e909
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256035
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>

subcmds/sync.py

@@ -576,8 +576,7 @@ later is required to fix a server side protocol bug.
         print('%s: Shared project %s found, disabling pruning.' %
               (project.relpath, project.name))
         if git_require((2, 7, 0)):
-          project.config.SetString('core.repositoryFormatVersion', '1')
-          project.config.SetString('extensions.preciousObjects', 'true')
+          project.EnableRepositoryExtension('preciousObjects')
         else:
           # This isn't perfect, but it's the best we can do with old git.
           print('%s: WARNING: shared projects are unreliable when using old '