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

rebase: only store collapse message once The message is determined by the user passing --message or --log when the rebase is started. There's no need to write it to a file for each rebased commit; writing it once at the start of the rebase is enough. Differential Revision: https://phab.mercurial-scm.org/D2712

rebase: collapse two nested if-conditions Also change the order since it feel to me like it's more about --collapse than it is about --keep. Differential Revision: https://phab.mercurial-scm.org/D2711

rebase: reduce scope of "dsguard" variables a bit Differential Revision: https://phab.mercurial-scm.org/D2710