Introduce a repository that contains some paths that were illegal
on PC-DOS circa 1981 (like `aux`, `con`, `com1`) and that in a
bizarre fit of retrocomputing, remain illegal on some "modern"
computers, despite being "new technology".

Introduce some aspirational tests that suggest that we should be
able to cope with trees and indexes that contain paths that
would be illegal on the filesystem, so that we can at least diff
them.  Further ensure that checkout will not write a repository
with forbidden paths.