fix: use scmutil.movedirstate() instead of reimplementing

I wrote this patch 2 years ago as a little cleanup. I wanted to
generally used `scmutil.movedirstate()` instead of manually updating
the dirstate because that is easy to get wrong. I didn't know until
today that the current code had a bug. So I added the test case two
patches before this one and dusted off this one patch. This is a
little slower than the previous code, as it diffs two
manifests. However, it fixes the bug and I don't think it's going to
be noticeably slower anyway.

Differential Revision: https://phab.mercurial-scm.org/D11210

hgext/fix.py

@@ -759,13 +759,12 b' def writeworkingdir(repo, ctx, filedata,'
     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)
 
     oldp1 = repo.dirstate.p1()
     newp1 = replacements.get(oldp1, oldp1)
     if newp1 != oldp1:
-        repo.setparents(newp1, nullid)
+        with repo.dirstate.parentchange():
+            scmutil.movedirstate(repo, repo[newp1])
 
 
 def replacerev(ui, repo, ctx, filedata, replacements):

tests/test-fix.t

@@ -370,7 +370,6 b' it clean.'
   M hello.whole
   $ hg fix -s . *
   $ hg st
-  M hello.whole (known-bad-output !)
   $ hg diff
 
   $ cd ..