fix(ignore, fs): allow loading ignore patterns behind symlink (fixes #10785) (#10786)

The recent change to disallow following symlinks by default conflicts
with the expected behavior of .stignore. This adds a new flag which may
be passed to OpenFile to skip default symlink-forbidding open flag.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs.go

@@ -240,16 +240,24 @@ func (f *BasicFilesystem) DirNames(name string) ([]string, error) {
 	return names, nil
 }
 
+// Open opens the file for reading, while not following symlinks
 func (f *BasicFilesystem) Open(name string) (File, error) {
 	return f.OpenFile(name, os.O_RDONLY, 0)
 }
 
+// OpenFile is the generalised file open call, using the flags to determine
+// the open semantics. We always add O_NOFOLLOW, disallowing opening files
+// by following symlinks, unless OptFollow is set.
 func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File, error) {
 	rootedName, err := f.rooted(name)
 	if err != nil {
 		return nil, err
 	}
-	flags |= alwaysOpenFlags // enforce extra bits in flags
+	if flags&OptFollow != 0 {
+		flags &^= OptFollow // unset the synthetic OptFollow bit
+	} else {
+		flags |= optNoFollow
+	}
 	fd, err := os.OpenFile(rootedName, flags, os.FileMode(mode))
 	if err != nil {
 		return nil, err
@@ -257,6 +265,8 @@ func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File,
 	return basicFile{fd, name}, err
 }
 
+// Create opens a file for writing, creating it as required. The Create will
+// fail if the destination is a symlink.
 func (f *BasicFilesystem) Create(name string) (File, error) {
 	return f.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o666)
 }