mercurial/filemerge.py

--- a/mercurial/filemerge.py	Thu Jan 04 21:36:58 2018 -0800
+++ b/mercurial/filemerge.py	Thu Jan 04 21:37:03 2018 -0800
@@ -636,7 +636,8 @@
         # merging in-memory, we must redirect the backup to the memory context
         # so we don't disturb the working directory.
         relpath = back[len(repo.wvfs.base) + 1:]
-        wctx[relpath].write(fcd.data(), fcd.flags())
+        if premerge:
+            wctx[relpath].write(fcd.data(), fcd.flags())
         return wctx[relpath]
     else:
         if premerge: