Improve performance of git status --ignored

Improve the performance of the directory listing logic when it wants to list
non-empty ignored directories. In order to show non-empty ignored directories,
the existing logic will recursively iterate through all contents of an ignored
directory. This change introduces the optimization to stop iterating through
the contents once it finds the first file. This can have a significant
improvement in 'git status --ignored' performance in repositories with a large
number of files in ignored directories.

For an example of the performance difference on an example repository with
196,000 files in 400 ignored directories:

| Command                    |  Time (s) |
| -------------------------- | --------- |
| git status                 |   1.2     |
| git status --ignored (old) |   3.9     |
| git status --ignored (new) |   1.4     |

Signed-off-by: Jameson Miller <jamill@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -49,7 +49,7 @@ struct cached_dir {
 static enum path_treatment read_directory_recursive(struct dir_struct *dir,
 	struct index_state *istate, const char *path, int len,
 	struct untracked_cache_dir *untracked,
-	int check_only, const struct pathspec *pathspec);
+	int check_only, int stop_at_first_file, const struct pathspec *pathspec);
 static int get_dtype(struct dirent *de, struct index_state *istate,
 		     const char *path, int len);
 
@@ -1404,8 +1404,13 @@ static enum path_treatment treat_directory(struct dir_struct *dir,
 
 	untracked = lookup_untracked(dir->untracked, untracked,
 				     dirname + baselen, len - baselen);
+
+	/*
+	 * If this is an excluded directory, then we only need to check if
+	 * the directory contains any files.
+	 */
 	return read_directory_recursive(dir, istate, dirname, len,
-					untracked, 1, pathspec);
+					untracked, 1, exclude, pathspec);
 }
 
 /*
@@ -1633,7 +1638,7 @@ static enum path_treatment treat_path_fast(struct dir_struct *dir,
 		 * with check_only set.
 		 */
 		return read_directory_recursive(dir, istate, path->buf, path->len,
-						cdir->ucd, 1, pathspec);
+						cdir->ucd, 1, 0, pathspec);
 	/*
 	 * We get path_recurse in the first run when
 	 * directory_exists_in_index() returns index_nonexistent. We
@@ -1793,12 +1798,20 @@ static void close_cached_dir(struct cached_dir *cdir)
  * Also, we ignore the name ".git" (even if it is not a directory).
  * That likely will not change.
  *
+ * If 'stop_at_first_file' is specified, 'path_excluded' is returned
+ * to signal that a file was found. This is the least significant value that
+ * indicates that a file was encountered that does not depend on the order of
+ * whether an untracked or exluded path was encountered first.
+ *
  * Returns the most significant path_treatment value encountered in the scan.
+ * If 'stop_at_first_file' is specified, `path_excluded` is the most
+ * significant path_treatment value that will be returned.
  */
+
 static enum path_treatment read_directory_recursive(struct dir_struct *dir,
 	struct index_state *istate, const char *base, int baselen,
 	struct untracked_cache_dir *untracked, int check_only,
-	const struct pathspec *pathspec)
+	int stop_at_first_file, const struct pathspec *pathspec)
 {
 	struct cached_dir cdir;
 	enum path_treatment state, subdir_state, dir_state = path_none;
@@ -1832,12 +1845,34 @@ static enum path_treatment read_directory_recursive(struct dir_struct *dir,
 			subdir_state =
 				read_directory_recursive(dir, istate, path.buf,
 							 path.len, ud,
-							 check_only, pathspec);
+							 check_only, stop_at_first_file, pathspec);
 			if (subdir_state > dir_state)
 				dir_state = subdir_state;
 		}
 
 		if (check_only) {
+			if (stop_at_first_file) {
+				/*
+				 * If stopping at first file, then
+				 * signal that a file was found by
+				 * returning `path_excluded`. This is
+				 * to return a consistent value
+				 * regardless of whether an ignored or
+				 * excluded file happened to be
+				 * encountered 1st.
+				 *
+				 * In current usage, the
+				 * `stop_at_first_file` is passed when
+				 * an ancestor directory has matched
+				 * an exclude pattern, so any found
+				 * files will be excluded.
+				 */
+				if (dir_state >= path_excluded) {
+					dir_state = path_excluded;
+					break;
+				}
+			}
+
 			/* abort early if maximum state has been reached */
 			if (dir_state == path_untracked) {
 				if (cdir.fdir)
@@ -2108,7 +2143,7 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
 		 */
 		dir->untracked = NULL;
 	if (!len || treat_leading_path(dir, istate, path, len, pathspec))
-		read_directory_recursive(dir, istate, path, len, untracked, 0, pathspec);
+		read_directory_recursive(dir, istate, path, len, untracked, 0, 0, pathspec);
 	QSORT(dir->entries, dir->nr, cmp_dir_entry);
 	QSORT(dir->ignored, dir->ignored_nr, cmp_dir_entry);