prune: handle branches that track missing branches

Series of steps:
* Create a local "b1" branch with `repo start b1` that tracks a remote
  branch (totally fine)
* Manually create a local "b2" branch with `git branch --track b1 b2`
  that tracks the local "b1" (uh-oh...)
* Delete the local "b1" branch manually or via `repo prune` (....)
* Try to process the "b2" branch with `repo prune`

Since b2 tracks a branch that no longer exists, everything blows up
at this point as we try to probe the non-existent ref.  Instead, we
should flag this as unknown and leave it up to the user to resolve.

This probably could come up if a local branch was tracking a remote
branch that was deleted from the server, and users ran something like
`repo sync --prune` which cleaned up the remote refs.

Bug: https://crbug.com/gerrit/11485
Change-Id: I6b6b6041943944b8efa6e2ad0b8b10f13a75a5c2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/236793
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Kirtika Ruchandani <kirtika@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>