Fix ManifestProject.partial_clone_exclude property.

Bug: b/256358360

Change-Id: Ic6e3a049aa38827123d0324c8b14157562c5986e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/353574


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Joanna Wang <jojwang@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>