Detach branch even when already on the latest revision using sync -d

This patch fixes repo behaviour when running sync -d with unmodified
topic branches.

Prior to this patch sync -d would see the latest revision is already
checked out, thus staying on the branch. Since "-d" means detach we
should follow git's behaviour and actually detach from the branch in
that case.

Basic test case - after a fresh repo init + sync -
        * repo start --all testdetach
        * repo sync -d
        * repo status
-> status shows active topic branch "testdetach",
   should show :
nothing to commit (working directory clean)

Change-Id: Ic1351e6b5721b76557a51ab09f9dd42c38a4b415
1 file changed