Add a user config for using git's external diff command for paging

[stefanhaller]

PR Description

This is similar to using lazygit's Git.Paging.ExternalDiffCommand config, except that the command is configured in git. This can be done either with git's diff.external config, or through .gitattributes, so it gives a bit more flexibility.

We could consider removing the Git.Paging.ExternalDiffCommand config now, because its functionality is covered by the new config. I decided to keep it though, because I don't want to make this a breaking change, and also because some users might want to have the external diff command only in lazygit but not on the command line.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 86934ce1	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (86934ce)	Report Missing	Report Missing	Report Missing
Head commit (40e9894)	57545	49988	86.87%

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 (#4832)	10	10	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%

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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. ↩

[rayluo]

PR Description

This is similar to using lazygit's Git.Paging.ExternalDiffCommand config, except that the command is configured in git. This can be done either with git's diff.external config, or through .gitattributes, so it gives a bit more flexibility.

We could consider removing the Git.Paging.ExternalDiffCommand config now, because its functionality is covered by the new config. I decided to keep it though, because I don't want to make this a breaking change, and also because some users might want to have the external diff command only in lazygit but not on the command line.

Took a quick glance. The rationale of introducing a new parameter while keeping the old one for backward compatibility makes sense.

Which panel(s) will pick up this new feature? I am not that familiar with lazygit's internal structure to know which panels the "commit.go", "diff.go", and "worktree.go" are affecting.

[stefanhaller]

Which panel(s) will pick up this new feature?

Files and Commits.

I just noticed that Stashes use neither the pager command nor the external diff command, I'll see if I can fix that as part of this PR.

[rayluo]

Yes, I have tested this feature branch. Indeed, a useExternalDiffGitConfig: true config will have lazygit honor git's existing external diff setup, in Files panel and in Commits panel. Ship it!

[stefanhaller]

@rayluo And now it should work for stashes too.

[rayluo]

@rayluo And now it should work for stashes too.

@stefanhaller And now it is also tested for stashes too. :-)

[OliverJAsh]

@stefanhaller After migrating from externalDiffCommand to useExternalDiffGitConfig, I'm no longer able to selectively stage hunks/lines in files. Is that expected or a bug? For example this is what happens when I hit enter on a file:

Screen.Recording.2025-09-14.at.10.04.42.mov