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>