tests: add test for issue 5494 but with --collapse
This was not fixed, so the test case currently demonstrates the
breakage.
Differential Revision: https://phab.mercurial-scm.org/D2714
tests: .hg/merge is a directory, so use `test -d`
This part of test-rebase-interrupts.t would have passed before the fix
in
a580b2d65ded (rebase: make sure merge state is cleaned up for no-op
rebases (
issue5494), 2017-05-18).
Differential Revision: https://phab.mercurial-scm.org/D2713