diffmanifests/sync: simplify repodir lookup

We have access to repodir on the command object itself, so we don't
need to pull it indirectly out of the manifest object.

Change-Id: I8688fb1c84979825efa966dc787e78c6f7ba3823
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/296542


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>