convert: avoid traceback in subversion sink stable

convert: avoid traceback in subversion sink

We've had a couple reports of subversion tracebacks that trigger when
the parents list is empty, and thus block showing what the commit
failure was on the next two lines.

--- a/hgext/convert/subversion.py	Fri Apr 24 15:29:02 2015 -0500
+++ b/hgext/convert/subversion.py	Mon Apr 27 11:11:26 2015 -0500
@@ -1293,7 +1293,7 @@
             try:
                 rev = self.commit_re.search(output).group(1)
             except AttributeError:
-                if not files:
+                if parents and not files:
                     return parents[0]
                 self.ui.warn(_('unexpected svn output:\n'))
                 self.ui.warn(output)