Mercurial's locate command will exit with status 1 when no files are

found. We use hg locate -r <REV> set:size(0) to locate empty files.

However, when there are no empty files, we will get None instead of

empty output because of Mercurial's status code. We now only try to

parse the list of files when there is output to avoid this issue.

Ran unit tests.
rbt post worked with this patch and didn't without it.