Allow gsb backup to combine with previous backups

Summary

Implements #34

gsb backup -c will combine any new changes with the previous backup
- and prompt for confirmation if that backup is tagged
gsb backup -cc will combine any new changes with all untagged backups since the last tagged backup (and will behave trivially if there are no new tagged backups)
Addressing some of the tech debt from #33, history.get_history has been refactored to separate the fetching from the logging
Fixed a bug where "rebased" commits weren't removing the trailing newline from the messages of the original commits
Adds usage docs for -c, -cc and also gsb delete
Switches the scope of the _repo_with_history fixture from module-level to session-level, which should be perfectly safe

I've tested out the prompts and logging messages I get when using the -c and -cc flags, and they look good to me.
After this PR is merged I'll be cutting a beta release to allow me to dogfood this new functionality (I see gsb -cc --tag "message" becoming my standard operating procedure).

[ ] This PR introduces a breaking change (will require a bump in the minor version)
[ ] The changes in this PR are high urgency and necessitate a hotfix or patch release (will require rebasing off of release)
[ ] This is a release (staging) PR (maintainer use only)

[x] I have read the contributor's guide
[x] I have run mkdocs serve locally and ensured that all API docs and changes I have made to the static pages are rendering correctly, with all links working
[x] All tech debt concerns have been resolved, documented as issues, or otherwise accepted
[x] I agree to license my contribution to this project under the GNU Public License v3