largefiles: rearrange how lfconvert detects non-local repos
Mainly this is so we can test that code; it has the side benefit of not
requiring network I/O to reject non-local repo URLs.
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd $(dirname $TESTDIR)
$ pyflakes mercurial hgext 2>&1 | $TESTDIR/filterpyflakes.py
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)