subrepo/svn: abort on commit with missing file (issue3029) Previous code was printing a traceback because it expected some error output from svn. But sometimes our definition of "changed" differs with the subversion one. For instance, subversion ignores missing files when committing. And when there are only missing files, svn commit will be a successful no-op with no output. Still, we should stick to our definition including missing files in changes as doing otherwise could cause surprising behaviour for the user.