tests: change the paths slightly in test-rebase-inmemory.t
c/c was a little difficult to understand (and verify that it was the *correct*
'c/' that was being talked about), and it's useful to have multiple directories
to prove that we are able to detect this even if there's no files (just a
subdirectory) in the immediate directory that's conflicting.
Differential Revision: https://phab.mercurial-scm.org/D5998