diff -r 2a7e777c9eed -r 45b678bf3a78 mercurial/merge.py
--- a/mercurial/merge.py	Wed Jan 17 16:52:13 2018 +0100
+++ b/mercurial/merge.py	Wed Jan 17 17:07:55 2018 +0100
@@ -1376,7 +1376,9 @@
                 if repo.wvfs.lexists(absf):
                     util.rename(absf, orig)
             wctx[f].clearunknown()
-            wctx[f].write(fctx(f).data(), flags, backgroundclose=True)
+            atomictemp = ui.configbool("experimental", "update.atomic-file")
+            wctx[f].write(fctx(f).data(), flags, backgroundclose=True,
+                          atomictemp=atomictemp)
             if i == 100:
                 yield i, f
                 i = 0