_get_revno only returns a commit count

* pbr/packaging.py(_get_revno): The _get_revno_and_last_tag method
always returns the same number of elements, so don't try to treat it
as an iterable and also rename it to something sensible.

Change-Id: I8a83135b979a13f8cd129126a29159e93c147467
Merge tag '0.10.7' into master

tag 0.10.7

Change-Id: I26f907a29ce94b10fe27c1365d99736d5180ded5