Support using the selected commit's message in a fixup

[jesseduffield]

PR Description

I had a spare couple hours so figured I'd whip up a PR for a feature I've wanted for a while.

I've optimised for muscle memory backwards compatibility here:

• Outside interactive rebase: press 'f' then instead of a confirmation panel, a menu appears where you can choose to keep the selected commit's message
• Inside interactive rebase: press 'f' then press 'c' to see the menu for keeping the message, where if you press 'c' again it will retain the current message. so 'fcc' is the chord to press.

We're also now showing the -C flag (which is what enables the behaviour) against the todo.

I've picked the 'c' keybinding because 'C' was taken and it corresponds to the flag. Previously that showed a warning about a change in keybinding for cherry picking but it's been ages since we've made that change so I'm happy to retire it.

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

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 39cd4741	✅ 96.41%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (39cd474)	Report Missing	Report Missing	Report Missing
Head commit (f317a97)	59607	51936	87.13%

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 (#5233)	167	161	96.41%

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%

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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. ↩

[stefanhaller]

This is similar to #4526 in many ways, except for the addition of the extra c command, which is a great idea.

My main problem with #4526 was that I found the new fixup menu (outside of rebase) more confusing than the old confirmation, so it slowed me down even though the muscle memory interaction stayed the same; that's probably better in your version because the top menu entry simply says "Fixup", which I like. I'll use this in production for a while and see how it feels.

I didn't look at the code very much yet, just a little bit of early feedback about the texts.

[stefanhaller]

This is very nice; I used it for a while now, and have no issues with the workflow. I even used the feature of using the upper commit's message once, so that's a welcome improvement.

I took the liberty to break up the large commit into a few smaller ones, I find this much easier to review.

[stefanhaller]

I usually prefer matching larger parts of a line instead of multiple Contains, mainly because it's more robust (you'd need a DoesNotContain("-C") in your version), but also because I find it easier to read.

Fixed in 897e01b.