fix: bulletproof external diff isolation (#380)#409
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tomasz-tomczyk merged 1 commit intomainfrom May 1, 2026
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## main #409 +/- ##
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+ Coverage 67.26% 67.32% +0.06%
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Files 26 26
Lines 9650 9665 +15
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+ Hits 6491 6507 +16
+ Misses 2649 2648 -1
Partials 510 510
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Some users report `--no-ext-diff` not suppressing their configured external diff tool (e.g. difftastic), causing crit to receive non-unified-diff output that breaks rendering. Belt-and-suspenders the disable: pass `-c diff.external=` to override the config at the command line, and strip GIT_EXTERNAL_DIFF / GIT_DIFF_OPTS from the subprocess environment. Both mechanisms run alongside the existing `--no-ext-diff` flag so any one of them is sufficient on its own. The env override is scoped to each git subprocess via cmd.Env — the parent process and shell environment are untouched.
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Summary
Issue #380 reports duplicate lines in the rendered diff. The reporter has `diff.external = difft` set and noted that `git diff --no-ext-diff` still produced difftastic output for them — meaning crit could be receiving non-unified-diff content that breaks `ParseUnifiedDiff`.
I couldn't reproduce the duplicate locally with a synthetic 3000-line .js fixture matching the screenshot pattern, even with `diff.external = difft` and `diff.suppressBlankEmpty = true`. So the root cause for that specific user is still unconfirmed — but their environment clearly has a path where `--no-ext-diff` alone isn't enough.
This PR makes external-diff disabling defense-in-depth:
If the duplicate truly stems from difft output leaking into crit's diff parser, this fixes it. If it doesn't, this is still correct hygiene and rules out one class of cause.
Test plan
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