Stash modes corrupt binary files (0 bytes) on partial commits

[alexey1312]

Description

Both stash = "patch-file" and stash = "git" modes corrupt binary files (PNG, JPG) when committing with partial staged changes. Binary files in the unstaged area become 0 bytes after the stash/unstash cycle.

Steps to Reproduce

1. Configure hk.pkl with stash enabled:

   ["pre-commit"] {
       fix = true
       stash = "patch-file"  // also reproduces with stash = "git"
       steps = linters
   }

2. Stage a Swift file change:

   echo "" >> SomeFile.swift
   git add SomeFile.swift

3. Modify a binary file but do not stage it:

   printf '\x00\x00\x00\x00' >> snapshot.png

4. Verify state:

   git status --short
   # M  SomeFile.swift        (staged)
   #  M snapshot.png           (unstaged)

5. Commit:

   git commit -m "test"

6. Check the binary file:

   wc -c < snapshot.png
   # 0    <-- file is now empty!

Expected Behavior

The unstaged binary file should retain its contents after the commit (same md5/size as before).

Actual Behavior

The unstaged binary file becomes 0 bytes (completely destroyed). md5 becomes d41d8cd98f00b204e9800998ecf8427e (empty file hash).

Tested Stash Modes

Mode	Result
"patch-file"	Binary file becomes 0 bytes
"git"	Binary file becomes 0 bytes
"none"	Binary file preserved (workaround)

Environment

• hk: v1.38.0
• macOS (Darwin 25.3.0, arm64)
• Git: standard macOS git
• Repository has .png snapshot test files in the working tree

Workaround

Using stash = "none" avoids the issue, but linters then see unstaged changes too.

[alexey1312]

Related discussions: #209 (stash behavior issues) and #234 (RFC for transactional stash). This report adds a specific data-loss scenario where binary files become 0 bytes.