export: flush the file pointer between patches

mercurial/cmdutil.py

@@ -699,6 +699,8 b" def export(repo, revs, template='hg-%h.p"
         for chunk in patch.diff(repo, prev, node, opts=opts):
             fp.write(chunk)
 
+        fp.flush()
+
     for seqno, rev in enumerate(revs):
         single(rev, seqno + 1, fp)