Use full name of the revision when checking dest-branch

The manifest usually doesn't sepecify the revision with the full name
(e.g. refs/heads/REV).
However, when checking if the name of the merge branch, full name is
used on the merge branch.

The CL use full name of revision when comparing it with the merge
branch.

Bug: b/370919047
Test: repo upload on a project with `dest-branch` set
Change-Id: Ib6fa2f7246beb5bae0a26a70048a7ac03b6c5a2f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/438401


Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Joe Hsu <joehsu@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>