commit: clear mergestate also with --amend (issue6304) The `hg commit --amend` uses the in-memory code, which naturally doesn't touch the merge state (well, it shouldn't anyway; I think I've fixed bugs in that area recently). We therefore need to clear the mergestate after calling `repo.commitctx()` since we expect that from `hg commit --amend`. Differential Revision: https://phab.mercurial-scm.org/D8932

tests: add test showing that merge state is not cleared by amend This is slightly modified version of the test case I provided in issue6304. Differential Revision: https://phab.mercurial-scm.org/D8931

requirements: introduce constants for `shared` and `relshared` requirements We add them to `WORKING_DIR_REQUIREMENTS` too as they should be stored in `.hg/requires` and have information about the type of working copy. Differential Revision: https://phab.mercurial-scm.org/D8926