export: serialize revisions to be exported per destination file

Prepares for porting to the formatter API, where we can't simply append
to existing files because JSON can't be streamed for example.

The modemap hack is removed since cmdutil.export() was the only user.
I also made the destination filename printed only once.

mercurial/cmdutil.py

@@ -1014,7 +1014,7 b' class _unclosablefile(object):'
     def __exit__(self, exc_type, exc_value, exc_tb):
         pass
 
-def makefileobj(ctx, pat, mode='wb', modemap=None, **props):
+def makefileobj(ctx, pat, mode='wb', **props):
     writable = mode not in ('r', 'rb')
 
     if isstdiofilename(pat):
@@ -1025,10 +1025,6 b" def makefileobj(ctx, pat, mode='wb', mod"
             fp = repo.ui.fin
         return _unclosablefile(fp)
     fn = makefilename(ctx, pat, **props)
-    if modemap is not None:
-        mode = modemap.get(fn, mode)
-        if mode == 'wb':
-            modemap[fn] = 'ab'
     return open(fn, mode)
 
 def openrevlog(repo, cmd, file_, opts):
@@ -1579,18 +1575,23 b' def _exportfntemplate(repo, revs, fntemp'
     """Export changesets to possibly multiple files"""
     total = len(revs)
     revwidth = max(len(str(rev)) for rev in revs)
-    filemode = {}
+    filemap = util.sortdict()  # filename: [(seqno, rev), ...]
 
     for seqno, rev in enumerate(revs, 1):
         ctx = repo[rev]
-        fo = makefileobj(ctx, fntemplate, mode='wb', modemap=filemode,
+        dest = makefilename(ctx, fntemplate,
                          total=total, seqno=seqno, revwidth=revwidth)
-        dest = fo.name
+        filemap.setdefault(dest, []).append((seqno, rev))
+
+    for dest in filemap:
+        with open(dest, 'wb') as fo:
+            repo.ui.note("%s\n" % dest)
         def write(s, **kw):
             fo.write(s)
-        repo.ui.note("%s\n" % dest)
-        _exportsingle(repo, ctx, match, switch_parent, seqno, write, diffopts)
-        fo.close()
+            for seqno, rev in filemap[dest]:
+                ctx = repo[rev]
+                _exportsingle(repo, ctx, match, switch_parent, seqno, write,
+                              diffopts)
 
 def export(repo, revs, fntemplate='hg-%h.patch', fp=None, switch_parent=False,
            opts=None, match=None):