Improve directory mtime support

[res2k]

This sets mtimes for directories that don't explicitly appear in the log, or only contain subdirectories, not files.
Implements some ideas mentioned in the comment block at the top.
May help with #47.

[MestreLion]

I like this, very much!!! Thanks a lot for digging into this!

Since I'm so busy at the moment it might take a few days (weeks?) for me to review and merge your code, but this is absolutely a very welcome feature!

[Repiteo]

Tested across a few larger repos, and everything seems to work as expected! Despite touched directories often increasing significantly, the actual processing time was functionally identical; the margin of error is so small that it was occasionally faster

[MestreLion]

I've skimmed over the code and yes, I believe performance is not impacted, the code was well written. But I'd still have to study both its implementation (there are some new key functions and even a new class), and also how his algorithm works conceptually, specially how it handles (and respects) things like pathspec, etc.

I said it could take weeks for me to do that, and here we are after months have passed. I'm truly, truly sorry. But I promise I'll dive into this before February as I want to release a new version before Ubuntu's 24.04 LTS feature freeze (and Debian import), as this will be my new daily driver workstation, so I want to make sure it ships with the latest git-restore-mtime