plumbing: gitignore, Fix loading of deeper ignored files

Fix loading of `.gitignore` files in nested ignored directories.
These were not matched in the current implementation,
as that only checks the ignores of the current directory.

As a side-effect this change also leads to significantly improved performance:
in a repository with 3M ignored files `Status` now takes 5 s instead of 27 s.

plumbing/format/gitignore/dir.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"bytes"
 	"io"
+	"log/slog"
 	"os"
 	"strings"
 
@@ -51,10 +52,15 @@ func readIgnoreFile(fs billy.Filesystem, path []string, ignoreFile string) (ps [
 // recursively traversing through the directory structure. The result is in
 // the ascending order of priority (last higher).
 func ReadPatterns(fs billy.Filesystem, path []string) (ps []Pattern, err error) {
-	ps, _ = readIgnoreFile(fs, path, infoExcludeFile)
+	return extendPatterns(fs, nil, path)
+}
 
+func extendPatterns(fs billy.Filesystem, start []Pattern, path []string) (ps []Pattern, err error) {
+	dirps, _ := readIgnoreFile(fs, path, infoExcludeFile)
 	subps, _ := readIgnoreFile(fs, path, gitignoreFile)
-	ps = append(ps, subps...)
+	dirps = append(dirps, subps...)
+	ignores := append(start, dirps...)
+	ps = dirps
 
 	var fis []os.FileInfo
 	fis, err = fs.ReadDir(fs.Join(path...))
@@ -64,12 +70,16 @@ func ReadPatterns(fs billy.Filesystem, path []string) (ps []Pattern, err error)
 
 	for _, fi := range fis {
 		if fi.IsDir() && fi.Name() != gitDir {
-			if NewMatcher(ps).Match(append(path, fi.Name()), true) {
+			if len(path) >= 2 && strings.HasPrefix(path[1], "output") {
+				slog.Debug("Bla")
+			}
+
+			if NewMatcher(ignores).Match(append(path, fi.Name()), true) {
 				continue
 			}
 
 			var subps []Pattern
-			subps, err = ReadPatterns(fs, append(path, fi.Name()))
+			subps, err = extendPatterns(fs, append(start, dirps...), append(path, fi.Name()))
 			if err != nil {
 				return
 			}