Begin commit messages with two empty lines.

This makes editors like Emacs wrap the commit message correctly. With
no empty line between the commit message and the 'HG:' lines, the
'HG:' lines would be wrapped as if they belonged to the commit message
and so lose their significance to Mercurial.

This makes no difference to what is stored by the commit -- whitespace
was already stripped from the beginning and end of commit messages.

mercurial/localrepo.py

@@ -879,6 +879,7 b' class localrepository(repo.repository):'
                 if text:
                     edittext.append(text)
                 edittext.append("")
+                edittext.append("") # Empty line between message and comments.
                 edittext.append(_("HG: Enter commit message."
                                   "  Lines beginning with 'HG:' are removed."))
                 edittext.append("HG: --")