mergestate: make clean() only be about creating a clean mergestate

This is similar to the previous patch, but moves the separation one
step further out, to `merge.py`.

Differential Revision: https://phab.mercurial-scm.org/D9034

mercurial/merge.py

@@ -1398,9 +1398,8 b' def applyupdates('
     _prefetchfiles(repo, mctx, mresult)
 
     updated, merged, removed = 0, 0, 0
-    ms = mergestatemod.mergestate.clean(
-        repo, wctx.p1().node(), mctx.node(), labels
-    )
+    ms = mergestatemod.mergestate.clean(repo)
+    ms.start(wctx.p1().node(), mctx.node(), labels)
 
     for f, op in pycompat.iteritems(mresult.commitinfo):
         # the other side of filenode was choosen while merging, store this in

mercurial/mergestate.py

@@ -177,12 +177,11 b' class mergestate(object):'
     statepathv2 = b'merge/state2'
 
     @staticmethod
-    def clean(repo, node=None, other=None, labels=None):
+    def clean(repo):
         """Initialize a brand new merge state, removing any existing state on
         disk."""
         ms = mergestate(repo)
         ms.reset()
-        ms.start(node, other, labels)
         return ms
 
     @staticmethod