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

tests/test-rebase-interruptions.t

@@ -461,5 +461,5 b' Make sure merge state is cleaned up afte'
   note: rebase of 1:fdaca8533b86 created no changes to commit
   saved backup bundle to $TESTTMP/repo/.hg/strip-backup/fdaca8533b86-7fd70513-rebase.hg
   $ hg resolve --list
-  $ test -f .hg/merge
+  $ test -d .hg/merge
   [1]