fix: rewrite writeworkingdir() to explicitly not work with merges

`hg fix` errors out early if there is an unfinished merge, so we
should have only one parent here. Making that explicit makes my next
patches simpler.

Differential Revision: https://phab.mercurial-scm.org/D11209

hgext/fix.py

@@ -132,6 +132,7 b' import subprocess'
 
 from mercurial.i18n import _
 from mercurial.node import (
+    nullid,
     nullrev,
     wdirrev,
 )
@@ -753,16 +754,18 b' def writeworkingdir(repo, ctx, filedata,'
 
     Directly updates the dirstate for the affected files.
     """
+    assert repo.dirstate.p2() == nullid
+
     for path, data in pycompat.iteritems(filedata):
         fctx = ctx[path]
         fctx.write(data, fctx.flags())
         if repo.dirstate[path] == b'n':
             repo.dirstate.set_possibly_dirty(path)
 
-    oldparentnodes = repo.dirstate.parents()
-    newparentnodes = [replacements.get(n, n) for n in oldparentnodes]
-    if newparentnodes != oldparentnodes:
-        repo.setparents(*newparentnodes)
+    oldp1 = repo.dirstate.p1()
+    newp1 = replacements.get(oldp1, oldp1)
+    if newp1 != oldp1:
+        repo.setparents(newp1, nullid)
 
 
 def replacerev(ui, repo, ctx, filedata, replacements):