tests: change the paths slightly in test-rebase-inmemory.t

Authored by spectral.

Description

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

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