rhg: Make `files` work on repo-relative paths when possible

rhg: Make `files` work on repo-relative paths when possible

When the current directory is outside of the repository we need to turn
everything into absolute filesystem paths in order to compute correct
relative paths.

This was previously done unconditionally, but is not necessary when
the current directory is inside the repository.

With this change `rhg files > /dev/null` at the root of a mozilla-central
snapshot goes from ~150 ms to ~70 ms. My repository is located at a
somewhat long path though (93 bytes). The effect may not be as pronounced
at a shorter path.

Differential Revision: https://phab.mercurial-scm.org/D10200

--- a/rust/rhg/src/commands/files.rs	Mon Apr 12 19:25:34 2021 -0700
+++ b/rust/rhg/src/commands/files.rs	Sat Mar 13 09:50:34 2021 +0100
@@ -53,21 +53,35 @@
     repo: &Repo,
     files: impl IntoIterator<Item = &'a HgPath>,
 ) -> Result<(), CommandError> {
-    let cwd = HgPathBuf::from(get_bytes_from_path(hg::utils::current_dir()?));
-    let working_directory = repo.working_directory_path();
-    let working_directory = current_dir()?.join(working_directory); // Make it absolute
-    let working_directory =
-        HgPathBuf::from(get_bytes_from_path(working_directory));
-
     let mut stdout = ui.stdout_buffer();
 
+    let cwd = current_dir()?;
+    let working_directory = repo.working_directory_path();
+    let working_directory = cwd.join(working_directory); // Make it absolute
+
     let mut any = false;
-    for file in files {
-        any = true;
-        let file = working_directory.join(file);
-        stdout.write_all(relativize_path(&file, &cwd).as_ref())?;
-        stdout.write_all(b"\n")?;
+    if let Ok(cwd_relative_to_repo) = cwd.strip_prefix(&working_directory) {
+        // The current directory is inside the repo, so we can work with
+        // relative paths
+        let cwd = HgPathBuf::from(get_bytes_from_path(cwd_relative_to_repo));
+        for file in files {
+            any = true;
+            stdout.write_all(relativize_path(&file, &cwd).as_ref())?;
+            stdout.write_all(b"\n")?;
+        }
+    } else {
+        let working_directory =
+            HgPathBuf::from(get_bytes_from_path(working_directory));
+        let cwd = HgPathBuf::from(get_bytes_from_path(cwd));
+        for file in files {
+            any = true;
+            // Absolute path in the filesystem
+            let file = working_directory.join(file);
+            stdout.write_all(relativize_path(&file, &cwd).as_ref())?;
+            stdout.write_all(b"\n")?;
+        }
     }
+
     stdout.flush()?;
     if any {
         Ok(())