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

[stefanhaller]

@OliverJAsh No, that's of course not expected, it's a silly bug. Please test the fix at #4895.

@rayluo How come you didn't notice this? Are you not actually using the feature yourself after all? (Not pointing fingers, I'm just surprised.)

[rayluo]

@rayluo How come you didn't notice this? Are you not actually using the feature yourself after all? (Not pointing fingers, I'm just surprised.)

• Of course I AM using the useExternalDiffGitConfig: true. After all, this was a feature that I wanted for 5 years. I even added that config into my own dot-files repo since the release of lazygit 0.55, so I am always using it since then.
• However, my test did not necessarily cover the "selectively stage hunks/lines in files" scenario. There are simply so many features in lazygit; I won't know which features would potentially be affected by useExternalDiffGitConfig: true, and I won't manually test all the lazygit features for the sake of testing. My test probably covered only the diff behavior itself.
• That being said, I tested the "selectively stage hunks/lines in files" scenario just now. "hit enter on a file", and then hit a or space all work as expected on my machine (which is running a 64-bit Linux). So, I am not sure what issue you folks are talking about.
• Last but not least, if a regression is considered too bad, I would suggest adding test cases into the code base. Currently, even the latest fix contains no test case. This regression might come back to bite us at a later time.

(Not making any excuse either. I am just equally surprised that this feature has a regression which I can't even reproduce; or perhaps I did not understand the bug report correctly.)

[stefanhaller]

• That being said, I tested the "selectively stage hunks/lines in files" scenario just now. "hit enter on a file", and then hit a or space all work as expected on my machine (which is running a 64-bit Linux). So, I am not sure what issue you folks are talking about.

It depends on the file type with your setup. Only those files that are covered by the "do-word-diff" diff driver will show the problem. Are you saying that it works even for those?

Anyway, no worries. Going to release a point update with the fix now.

[rayluo]

It depends on the file type with your setup. Only those files that are covered by the "do-word-diff" diff driver will show the problem. Are you saying that it works even for those?

Oh I see. Sorry my previous test did not cover that scenario. (For what it's worth, I was using the external diff for word diff effect, which is only useful for - thus only configured on - text files such as *.md and *.txt etc.; but I am not writing docs recently so I did not use "selectively stage hunks" for my .md files.)

I tested it just now. The 0.55.0 was indeed buggy and the 0.55.1 fixed it. Thanks again to you and the community for making lazygit better and better over time!