Smart stashing

[whatisaphone]

Smart stashing does not seem to work as described in the docs. Here's a repro:

mise.toml:

[tools]
hk = "1.54.0"
prettier = "3.9.6"

hk.pkl:

amends "package://github.com/jdx/hk/releases/download/v1.54.0/hk@1.54.0#/Config.pkl"
import "package://github.com/jdx/hk/releases/download/v1.54.0/hk@1.54.0#/Builtins.pkl"

local linters = new Mapping<String, Step> {
    ["prettier"] = Builtins.prettier
}

hooks {
    ["pre-commit"] {
        fix = true
        stash = "git"
        stage = false
        fail_on_fix = true
        steps = linters
    }
}

Repro steps:

git init
echo $'const x = 5;\nconst y = 6;\nconst z = 7;' > foo.js
git add .
git commit -minitial

sed -i 's/x = 5/x=1111/' foo.js
git add .
sed -i 's/z = 7/z=2222/' foo.js
# now foo.js is partially staged.

hk run pre-commit

After running hk, nothing changed in either the worktree or git's index. According to the docs ("3. After hooks run"), I'd expect the changes from prettier to be merged into the worktree, so the x line is formatted, but the z line remains unformatted. Could this be a bug or did I misunderstand the docs?

[jdx]

Confirmed: this is a bug, not a documentation mismatch. When stage=false (also enforced by fail_on_fix=true) disables auto-staging, stash restoration used the unchanged index as the fixer side of its three-way merge. That made the formatter change appear absent, so it was discarded while restoring the unstaged edit.

I opened PR #1145, which snapshots the isolated post-fixer worktree when staging is disabled, merges it with the saved unstaged edits, and leaves the original index untouched. It includes the exact partially staged single-file regression and passes with both libgit2 and shell Git.

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