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