project: Copy and link files even with local branches

In the winding maze that constitutes Sync_LocalHalf(), there are paths
in which we don't copy-and-link files. Examples include something like:

  cd some/project/
  repo start head .
  # do some work, make some commit, upload that commit to Gerrit

  [[ ... in the meantime, someone addes a <linkfile ...> for
     some/project/ in the manifest ... ]]

  cd some/project/
  git pull --rebase
  repo sync

In this case, we never hit a `repo rebase` case, which might have saved
us. Instead, the developer is left confused why some/project/ never had
its <linkfile>s created.

Notably, this opens up one more corner case in which <linkfile ... /> or
<copyfile ... /> could potentially clobber existing work in the
destination directory, but there are existing cases where that's true,
and frankly, those seem like bigger holes than this new one.

Change-Id: I394b0e4529023a8ee319dc25d03d513a19251a4a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/437421
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Brian Norris <briannorris@google.com>
Commit-Queue: Brian Norris <briannorris@google.com>

project.py

@@ -1694,6 +1694,8 @@ class Project:
                             project=self.name,
                         )
                     )
+                    return
+                syncbuf.later1(self, _doff, not verbose)
                 return
             elif pub == head:
                 # All published commits are merged, and thus we are a