When writing manifest, set the dest-branch attribute for projects

When generating a revision locked manifest, we need to know what
ref to push changes to when doing 'repo upload'.  This information
is lost when we lock the revision attribute to a particular commit
hash, so we need to expose it through the dest-branch attribute.

Bug: https://crbug.com/1005103
Test: manual execution
Change-Id: Ib31fd77ad8c9379759c4181dac1ea97de43eec35
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/263572
Tested-by: Sean McAllister <smcallis@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>