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>
adding a
adding b
copy .hg/patches to .hg/patches.1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
M b
created new head
a
b
merging with queue at: .hg/patches.1
applying rm_a
now at: rm_a
b
popping rm_a
popping .hg.patches.merge.marker
patch queue now empty
% init t2
adding a
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% create the reference queue
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
% merge
merging with queue at refqueue
applying patcha
now at: patcha