hg status: do not crash when there is no .hgignore and a directory is not readable

If a repo has no .hgignore (this happens frequently on repositories that come from a git clone, which tend to have a .gitignore but no .hgignore), and in the working copy there is a directory which is not readable by the current user (for example, for a ownership or persmission problem), hg-git causes a crash in hg status instead of printing an error message.

An almost complete stack trace is:

$ hg status
...
Traceback (most recent call last):
  File "/opt/mercurial/hg-git/hggit/gitdirstate.py", line 99, in _ignore
    fp = open(files[0], 'rb')
FileNotFoundError: [Errno 2] No such file or directory: b'/tmp/example/.hgignore'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/mercurial/hg-git/hggit/gitdirstate.py", line 182, in _finddotgitignores
    entries = util.listdir(join(nd), stat=True, skip=skip)
PermissionError: [Errno 13] Permission denied: '/tmp/example/not_readable'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  [...]
  File "/opt/mercurial/hg-git/hggit/gitdirstate.py", line 102, in _ignore
    fns = self._finddotgitignores()
  File "/opt/mercurial/hg-git/hggit/gitdirstate.py", line 185, in _finddotgitignores
    fwarn(nd, inst.strerror)
  File "/opt/mercurial/hg-git/hggit/gitdirstate.py", line 128, in fwarn
    self._ui.warn(b'%s: %s\n' % (self.pathto(f), msg))
TypeError: %b requires a bytes-like object, or an object that implements __bytes__, not 'str'

After this change, the output of hg status under such condition becomes:

$ hg status
not_readable: Permission denied
not_readable: Permission denied <-- duplicated!
? .gitignore
? dir/.gitignore
? dir/bar
? foo
? foobar

The "Permission denied" message is printed twice. This is probably a pre-existing bug that was masked by the crash and is now uncovered.

Tested on:

• Ubuntu 20.04
• Python 3.8.5
• mercurial 5.6.1
• hg-git 6e576b67d1ce.