extract get_version() function
extract main() function
extract function parse_args()
extract get_repocachedir() method
add info and caveats about data persisting between tests
extract _check_changes() method for reuse
extract _new_change_entry_regexp() method for reuse
gittests: give default author test a unique name

This test probably wasn't being run before :-/
change default git versionformat to %ct.%h
fix changes generation when version is blank
update to 0.5.0

The change of default versionformat for git is sort of
backwards-incompatible, although not an API change.