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

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