problems with nmbug and empty prefix

[David Bremner]

Currently nmbug doesn't seem to work with an empty prefix for me.

╭─ zancas:~ 
╰─% bash
bremner at zancas:~$ export NMBGIT=/tmp/nmbug
bremner at zancas:~$ export NMBPREFIX=""
bremner at zancas:~$ nmbug commit
/usr/lib/python2.7/urllib.py:1303: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  return ''.join(map(quoter, s))
Error flushing output: <fd:8>: Broken pipe
[u'notmuch', u'dump', u'--format=batch-tag', u'--', u'<censored>'] exited with 254

where <censored> is the complete list of tags in my database.

/tmp/nmbug is a previously cloned repo.

I'm not sure if this is because of some problem with a null prefix
specifically, or just the number of messages involved.