Before this change, Bisync did not check to make sure that Path1 and Path2 do
not overlap, nor did it check for overlaps with `--backup-dir`. While `sync`
does check for these things, it can sometimes be fooled because of the way
Bisync calls it with `--files-from` filters. Relying on sync could also leave a
run in a half-finished state if it were to error in one direction but not the
other (`--backup-dir` only checks for overlaps with the dest.)
After this change, Bisync does its own check up front, so we can quickly return
an error and exit before any changes are made.
