Only attempt to verify heads hg has, too

Only attempt to verify heads hg has, too

In the git repo there may be any number branches that are not hg
imported branches, so it doesn't make sense to print warnings when a
non-hg head isn't at what it was last time.

Now we get a list of branchtags hg has and only verify these.

Signed-off-by: Rocco Rutte <pdmef@gmx.net>