evolve: fix stabilization of orphan merge changesets in case of conflicts As demonstrated by tests added in previous patch, if one of the parent of a merge changeset has a new version and merge changeset gets, running `hg evolve` to stabilize that if results in conflicts will lead to lose of other parent of that merge changeset when we resolve conflicts and do `hg evolve --continue`. This patch fixes the behavior, by checking whether we are continuing evolving a merge changeset and if yes, setting the dirstate parents according to that.

tests: add a test file of resolving orphaness of merge changesets This patch adds a new test file named test-evolve-orphan-merge.t which contains test cases when `hg evolve` try to stabilize a merge changeset which was orphan. This file contains 20-30% of all the cases as we have got something to fix before we add tests for rest of the cases. Going by this approach will make sure we don't live unfixed cases in the test file and the test files does not become a pile of hacks which are used to get in state when we fix the bugs.

grab: gracefully handle the case when we try to grab parent of wdir Before this patch, we go all the way down to obsmarker creation logic and gets an error saying that changeset cannot obsolete itself. We should be more smart here and error out early saying we cannot grab that changeset.