handle --force when checking for incorrect password

[andreineculau]

compare dirty before and after. see #195 (comment)

PS: switched to the same dirty check we have in run_safety_checks for consistency

[jmurty]

This looks like a good improvement thanks, but I'm getting test failures in cases where a newly-created repository doesn't yet have a HEAD reference:

fatal: bad revision 'HEAD'

We will need to find a variant of the dirty files check command that works in this scenario.

Note that I moved the duplicated dirty file check command into a reusable function list_dirty_files() so it only needs to be adjusted in one place.