rust-dirstatemap: remove additional lookups in dirstatemap
We're copying this shortcut from the Python implementation, pretty standard
for this codebase.
Differential Revision: https://phab.mercurial-scm.org/D7117
py3: fix sorting of obsolete markers during push
This fixes similar errors as in 01e8eefd9434:
TypeError: '<' not supported between instances of 'NoneType' and 'tuple'