rhg: test `rhg files --config ui.relative-paths ...` Add some tests. All of these are falling back for now, will be fixed in follow-up commits.

cleanup: simplify code

dirstate: better error messages when dirstate is corrupted The current error message "Overflow in dirstate" sounds confusing because it suggests either a certain size limit that's being exceeded, or integer arithmetic overflowing. The reality is just a file being shorter than expected.