Pass --no-ext-diff to git so GIT_EXTERNAL_DIFF doesn't break parsing

[icholy]

When GIT_EXTERNAL_DIFF is set (e.g. difftastic), git diff emits the external tool's output instead of unified diff format. The parser sees no recognizable hunks and the UI shows 'No changes found'. Disable the external diff driver for all internal diff calls.

[cschanhniem]

The fix needs --no-ext-diff added to every git diff invocation in packages/git/src/diff.ts. Four call sites need updating:

1. getDiff() (line ~2): Change git diff to git diff --no-ext-diff in the cmd construction
2. getDiffStat() (line ~67): Add --no-ext-diff after git diff and before --stat
3. getDiffFiles() (line ~52): Add --no-ext-diff to the git diff --name-only command
4. getUntrackedDiff() (line ~16): Add --no-ext-diff to the git diff --no-index command

The simplest approach is to add --no-ext-diff directly in the getDiff() and getDiffStat() functions since they are the central diff entry points. The getUntrackedDiff and getDiffFiles functions call git diff directly so they need separate changes.

Would you like me to open a PR with these changes?

[icholy]

... this is a PR with those changes

[cschanhniem]

Right you are — the PR is already up. Thanks for pointing out the redundancy.