Preserve bzr commit metadata (as commit message trailers).

XXX: Use git notes instead of appending trailers to commit messages!
XXX: Does not preserve dotted revnos of merged unsquashed commits.

diff -ur fastimport-virgin/exporter.py fastimport/exporter.py
--- fastimport-virgin/exporter.py	2017-03-14 08:30:30.000000000 -0600
+++ fastimport/exporter.py	2017-03-14 13:23:56.000000000 -0600
@@ -319,7 +319,7 @@
         mark = 1
         self.revid_to_mark[revid] = mark
         file_cmds = self._get_filecommands(bzrlib.revision.NULL_REVISION, revid)
-        self.print_cmd(self._get_commit_command(ref, mark, revobj, file_cmds))
+        self.print_cmd(self._get_commit_command(ref, mark, revobj, file_cmds, revid));
 
     def emit_commit(self, revid, ref):
         if revid in self.revid_to_mark or revid in self.excluded_revisions:
@@ -355,7 +355,8 @@
         mark = ncommits + 1
         self.revid_to_mark[revid] = mark
         file_cmds = self._get_filecommands(parent, revid)
-        self.print_cmd(self._get_commit_command(ref, mark, revobj, file_cmds))
+        #sys.stderr.write("parent revid: %s %s\n" % (parent, revid))
+        self.print_cmd(self._get_commit_command(ref, mark, revobj, file_cmds, revid));
 
         # Report progress and checkpoint if it's time for that
         self.report_progress(ncommits)
@@ -377,7 +378,15 @@
             name, email = parseaddr(user)
         return name.encode("utf-8"), email.encode("utf-8")
 
-    def _get_commit_command(self, git_ref, mark, revobj, file_cmds):
+    def _revid_to_revno(self, revid):
+        # XXX: The callers do not know how to get revno for merged revisions
+        # because those revisions do not belong to self.branch.
+        try:
+            return self.branch.revision_id_to_revno(revid);
+        except:
+            return None;
+
+    def _get_commit_command(self, git_ref, mark, revobj, file_cmds, revid):
         # Get the committer and author info
         committer = revobj.committer
         name, email = self._get_name_email(committer)
@@ -427,6 +436,63 @@
         # don't repeat it here.
         if self.plain_format:
             properties = None
+            trailer = ""
+            revno = self._revid_to_revno(revid);
+            if revno: 
+                trailer += ("bzr::revno: %s\n" % revno);
+            #sys.stderr.write("properties: %s\n" % revobj.properties)
+            if not revobj.properties["branch-nick"]:
+                raise RuntimeError("missing branch-nick in %s" % revobj.properties);
+            for propName, propValue in revobj.properties.items():
+                if propName == "author":
+                    raise RuntimeError("unexpected author property: %s\n" % revobj.properties)
+
+                elif propName == "authors":
+                    # handled further below using pre-computed more_authors
+                    if not more_authors and len(propValue.split("\n")) > 1:
+                        raise RuntimeError("missing more_authors for authors property: %s\n" % revobj.properties)
+
+                elif propName == "branch-nick":
+                    # TODO: Exclude HEAD and trunk?
+                    trailer += ("bzr::branch-nick: %s\n" % propValue)
+
+                elif propName == "bugs":
+                    # https://bugs.launchpad.net/bzr-fastimport/+bug/1606973
+                    # property bugs 54 http://bugs.squid-cache.org/show_bug.cgi?id=3728 fixed
+                    fixed = []
+                    for bugstatement in propValue.split("\n"):
+                        bugurl, comment = bugstatement.split()
+                        if not bugurl.startswith("http://bugs.squid-cache.org/show_bug.cgi?id="):
+                            raise RuntimeError("unknown bug url: %s\n" % bugurl)
+                        if comment == "fixed":
+                            fixed.append(bugurl)
+                        else:
+                            raise RuntimeError("unknown bug action: %s\n" % comment)
+
+                    if not fixed:
+                        raise RuntimeError("empty bugs: %s\n" % bugstatement)
+
+                    trailer += ("bzr::fixes: %s\n" % ', '.join(fixed))
+
+                elif propName == "rebase-of":
+                    continue # TODO: Anything we can do about it?
+
+                else:
+                    # XXX: we need to iterate and check all properties!
+                    raise RuntimeError("unknown property: %s\n" % revobj.properties)
+
+            if more_authors:
+                for auth in more_authors:
+                    trailer += ("bzr::co-author: %s" % auth)
+
+            if trailer:
+                # separate commit message and trailer with exactly one empty line,
+                # assuming the commit message has no, one, or two new lines.
+                if not revobj.message.endswith("\n"):
+                    revobj.message += "\n\n";
+                elif not revobj.message.endswith("\n\n"):
+                    revobj.message += "\n";
+                revobj.message += trailer;
         else:
             properties = revobj.properties
             for prop in self.properties_to_exclude: