Add fallback to detecting main branches for fresh repositories

[ChrisMcD1]

PR Description

All other attempts to determine the main branch fail on a fresh repository right after git init because there are no actual commits that these branches point to.

Fixes #5058

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[ChrisMcD1]

Converted to draft while I fix up the test failures

[stefanhaller]

I don't think this is the best approach; it doesn't address the root cause. There are other situations where the main branches get stale, for example if you rename your master branch to main; you'll have to restart lazygit for it to pick up that change, too. I'd prefer a solution that takes care of all these situations.

The real issue is that we cache the determined main branch refs and then never forget them again (unless the user config changes). The challenge is to find the right conditions under which to drop the cache; an easy and straightforward solution would be to drop the cache at the beginning of refresh whenever the refresh scope includes commits or branches, but I'm not sure we want to afford that. Maybe it's good enough (it won't affect the timing of starting up lazygit, which is the most important), but we'd have to take some measurements to see if we want to incur the cost on every refresh.

Investigating this didn't have high priority for me so far because the situations in which it is buggy are so rare, and easily fixed by restarting lazygit.