fix: keep native Windows paths in v2 git patches (#1492)

ACP v2 now says each `git_patch` path, once unquoted, is the same native
absolute path string as the matching `changes` entry
(agentclientprotocol/agent-client-protocol#2650).

The v2 form converted `C:\work\App.ts` to `C:/work/App.ts`, so on
Windows the patch and `changes[].path` named different strings. The v2
form now keeps the native path, which Git quoting writes as
`"C:\\work\\App.ts"`. Git's own form, which the AIR extension uses,
still uses forward slashes and the `a/`/`b/` prefixes.

src/diff.ts

@@ -89,7 +89,7 @@ export type PatchForm = "air" | "v2";
 /**
  * How a git patch names its file. `git` is git's own form: the leading slash
  * dropped and the `a/` and `b/` prefixes added. `absolute` is the form of an
- * ACP v2 `git_patch`, whose paths must be absolute: the path itself.
+ * ACP v2 `git_patch`, whose paths must be absolute: the native path itself.
  */
 type GitPatchPaths = "git" | "absolute";
 
@@ -438,11 +438,11 @@ export function gitPatchText(
 }
 
 function gitPatchHeader(filePath: string, change: FileChange, paths: GitPatchPaths): string[] {
-  // A Windows path gets forward slashes so the header names the same file on
-  // every platform.
-  const slashed = filePath.replaceAll("\\", "/");
-  // git drops one leading slash of an absolute path.
-  const name = paths === "git" ? slashed.replace(/^\/+/u, "") : slashed;
+  // git's form uses forward slashes, so the header names the same file on
+  // every platform, and drops one leading slash of an absolute path. An ACP v2
+  // patch keeps the native path so that, once unquoted, it equals the `changes`
+  // entry: `C:\work\App.ts` is written `"C:\\work\\App.ts"`.
+  const name = paths === "git" ? filePath.replaceAll("\\", "/").replace(/^\/+/u, "") : filePath;
   const oldName = quoteGitPath(paths === "git" ? "a/" : "", name);
   const newName = quoteGitPath(paths === "git" ? "b/" : "", name);
   // git ends a ---/+++ name that contains a space with a tab, for GNU patch.

src/tests/diff-patch.test.ts

@@ -466,6 +466,12 @@ describe("ACP v2 diffs", () => {
     );
   });
 
+  it("keeps a Windows path native in a git patch, so it matches the changes entry", () => {
+    expect(gitPatchText("C:\\work\\App.ts", "update", [hunk], "absolute")).toContain(
+      'diff --git "C:\\\\work\\\\App.ts" "C:\\\\work\\\\App.ts"\n--- "C:\\\\work\\\\App.ts"\n+++ "C:\\\\work\\\\App.ts"\n',
+    );
+  });
+
   it("shows no diff at tool use, where the input holds a snippet or no old text", () => {
     const renderer = new AcpToolCallRenderer(v2);
     const edit = { file_path: "/work/a.ts", old_string: "old", new_string: "new" };