Tests: Make index_mtime_change_invalidates_cache deterministic

`index_mtime_change_invalidates_cache` used to call `git add` and then `sleep(1100 ms)` so the FS-resolution lottery didn't squash the mtime change on overlayfs (CI's second-resolution timestamps). Worked, but spent ~1.3 s of pure wall-clock per run and the test was the slowest in the git module.
- Replaced the sleep with an explicit `filetime::set_file_mtime` bump on `.git/index` by +2 s after the `git add`. `list_status`'s cache check keys off that exact mtime, so the test exercises the same code path with no FS-quirks dependency.
- Now runs in 0.117 s (was ~1.3 s).

Author: vdavid

apps/desktop/src-tauri/src/file_system/git/status.rs

@@ -464,14 +464,22 @@ mod cache_tests {
             "fresh repo had no untracked new.txt"
         );
 
-        // Stage a new file. `git add` rewrites `.git/index`, bumping the mtime.
+        // Stage a new file. `git add` rewrites `.git/index`, but on filesystems
+        // with second-resolution timestamps (CI overlayfs is the canary), the
+        // new mtime may equal the pre-add mtime and the cache stays warm.
+        // Instead of sleeping a full second to wait the FS out (the previous
+        // shape did `sleep(1100ms)`, ~1.3 s of pure wall-clock per run),
+        // forcibly bump the index mtime via `filetime`. This is the exact
+        // invariant `list_status`'s cache check keys off, so we're testing the
+        // same code path — minus the FS-resolution lottery.
         std::fs::write(dir.join("new.txt"), "x\n").unwrap();
         run(&dir, &["add", "new.txt"]);
 
-        // Sleep one filesystem tick so the mtime is guaranteed to change on
-        // filesystems with second-resolution timestamps. macOS APFS has
-        // sub-second resolution but CI's overlayfs sometimes doesn't.
-        std::thread::sleep(std::time::Duration::from_millis(1100));
+        let index_path = root.join(".git").join("index");
+        let cur_mtime = std::fs::metadata(&index_path).unwrap().modified().unwrap();
+        let bumped = cur_mtime + std::time::Duration::from_secs(2);
+        filetime::set_file_mtime(&index_path, filetime::FileTime::from_system_time(bumped))
+            .expect("bump .git/index mtime");
 
         let second = list_status(&handle, &dir).unwrap();
         let entries_second: Vec<&str> = second.iter().map(|e| e.relative_path.as_str()).collect();