Show an error when checking out a file would overwrite local modifications

[stefanhaller]

Checking out a file discards all local modifications to that file; checking out a directory discards modifications to all files in that directory. This can result in lost work if the user isn't aware of this.

Instead of showing a confirmation (to which the user might press enter too hastily, still losing their work), or showing a menu with auto-stash/discard options, we simply show an error. The assumption is that this is an unusual situation that users won't run into often, and if they do, it's easy enough to manually address it by stashing or discarding the changes as appropriate.

Fixes #5142.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for d34266d1	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d34266d)	Report Missing	Report Missing	Report Missing
Head commit (3ccd33b)	59389	51716	87.08%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#5154)	48	48	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

See your quality gate settings    Change summary preferences

Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩

[KJ7LNW]

I wonder if it should be a super trivial feature that just completely blocks the user and tells them "Checkout is blocked by existing modifications, address those first"

I think this case is unusual enough that if they accidentally hit c in that directory, we don't want them to lose what they're working on even if they press enter. I'm suggesting this because I find my lazygit muscle memory sometimes too fast to press Enter because in certain key sequences I'm used to forcing an override of something.

Perhaps they should be forced to navigate to 2 and properly manage files in the way that they are normally used to doing (s, d, etc...)

This is also probably the simplest implementation and easiest to release.

[stefanhaller]

@KJ7LNW Fair enough. Please review.

[KJ7LNW]

I built this PR and tested it directly. It seems to work well, and I like the implementation as well. Feel free to merge when ready!

-Eric