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.

largefiles: test lfconvert error handling; remove redundant code

largefiles: extract test-lfconvert.t from test-largefiles.t test-largefiles.t is getting pretty big, and there's lots more to test in lfconvert. And lfconvert is a fairly orthogonal feature to the rest of largefiles.