convert/svn: make sink recover gracefully from empty changeset
Otherwise when processing a changeset that in fact changes no files
(perhaps due to bug in import from CVS) can get something like:
unexpected svn output:
abort: unable to cope with svn output
Bug report and patch draft by Jesse Glick <jesse.glick@sun.com>
Checked out revision 0.
A a
Adding a
Transmitting file data .
Committed revision 1.
% initial roundtrip
scanning source...
sorting...
converting...
0 added a
scanning source...
sorting...
converting...
% second roundtrip should do nothing
scanning source...
sorting...
converting...
scanning source...
sorting...
converting...
% new hg rev
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% adding an empty revision
% echo hg to svn
scanning source...
sorting...
converting...
1 b
0 emtpy
% svn back to hg should do nothing
scanning source...
sorting...
converting...
% hg back to svn should do nothing
scanning source...
sorting...
converting...