largefiles: beef up test-lfconvert.t so it's more thorough
- instead of converting a single changeset with a single file, convert
two changesets which each add a mix of normal and large files
- make one of the large files not-so-large, but identified as large
by filename
- use shorter, simpler filenames
- put some files in a subdirectory
- test that file contents are correctly converted
- drop unnecessary cleanup step