filemerge: flush if using deferred writes when running a merge tool

Since merge tools might read from the filesystem, we need to write out our
deferred writes here.

No-ops if not using deferred writes.

Differential Revision: https://phab.mercurial-scm.org/D627

mercurial/filemerge.py

@@ -666,6 +666,11 b' def _filemerge(premerge, repo, wctx, myn'
         onfailure = _("merging %s failed!\n")
         precheck = None
 
+        # If using deferred writes, must flush any deferred contents if running
+        # an external merge tool since it has arbitrary access to the working
+        # copy.
+        wctx.flushall()
+
     toolconf = tool, toolpath, binary, symlink
 
     if mergetype == nomerge: