filemerge: only write in-memory backup during premerge

This wasn't broken, but should mirror the non-in memory case to save an extra
write.

Differential Revision: https://phab.mercurial-scm.org/D1807

mercurial/filemerge.py

@@ -636,6 +636,7 b' def _makebackup(repo, ui, wctx, fcd, pre'
         # 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:]
+        if premerge:
         wctx[relpath].write(fcd.data(), fcd.flags())
         return wctx[relpath]
     else: