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.