bit_remote: resolve_gitdir treats absolute Windows drive paths as relative

[Milky2018]

Summary

mizchi/bit_remote 0.48.0 resolves an absolute Windows gitdir: target as a relative path. For example:

• .git path: C:/linked/.git
• Contents: gitdir: C:/repo/.git/worktrees/linked
• Expected: C:/repo/.git/worktrees/linked
• Actual: C:/linked/C:/repo/.git/worktrees/linked

The published package reproduces this deterministically. Current main at 2681c3bc815d01aa56e678f45eb98e268c052481 still checks only whether the target starts with /.

Minimal reproduction

This test injects Windows-style .git contents through the public RepoFileSystem trait, so it runs without a Windows host or any actual repository. Create these files and run moon test --target native.

moon.mod

name = "repro/bit-gitdir"
version = "0.0.0"
import {
  "mizchi/bit_remote@0.48.0",
  "mizchi/bit_types@0.48.0",
}

moon.pkg

import {
  "mizchi/bit_remote" @remote,
  "mizchi/bit_types" @types,
  "moonbitlang/core/encoding/utf8",
} for "wbtest"

repro_wbtest.mbt

///|
priv struct GitdirFixture { target : String }

///|
impl @types.RepoFileSystem for GitdirFixture with fn read_file(self, _) {
  @utf8.encode("gitdir: " + self.target + "\n")
}

///|
impl @types.RepoFileSystem for GitdirFixture with fn readdir(_, _) { [] }

///|
impl @types.RepoFileSystem for GitdirFixture with fn is_dir(_, _) { false }

///|
impl @types.RepoFileSystem for GitdirFixture with fn is_file(_, _) { true }

///|
impl @types.RepoFileSystem for GitdirFixture with fn mtime(_, _) { (0, 0) }

///|
test "absolute Windows gitdir" {
  let target = "C:/repo/.git/worktrees/linked"
  let fs = GitdirFixture::{ target }
  assert_eq(@remote.resolve_gitdir(fs, "C:/linked/.git"), target)
}

Actual assertion failure:

"C:/linked/C:/repo/.git/worktrees/linked" != "C:/repo/.git/worktrees/linked"

Cause and suggested coverage

resolve_gitdir treats every target not starting with / as relative and prepends the .git file's parent. detect_git_dir uses that result, so library callers can receive an invalid metadata directory for a Windows linked worktree.

Please use platform-aware absolute-path and parent-path handling. Regression coverage should include Windows drive-rooted paths, native-backslash/UNC forms, existing POSIX absolute paths, and ordinary relative gitdir: paths. Drive-relative paths such as C:relative should remain distinct from C:/absolute.

Verification scope

• Executed the injected-filesystem reproduction on macOS against mizchi/bit_remote@0.48.0 and mizchi/bit_types@0.48.0.
• Confirmed the unchanged resolver in current upstream source.
• I have not run a real linked-worktree integration test on Windows. The Windows end-to-end impact is inferred from the reproduced path-resolution result.
• Toolchain: moon 0.1.20260920 (914d7da), moonc v0.10.14+7d59c7ec9.