perf: unbust perfbdiff --alldata
This broke in f84fc6a92817 due to a refactored manifest API.
The fix is a bit hacky - perfbdiff doesn't yet support tree manifests
for example. But it gets the job done.
A test has been added for --alldata so this doesn't happen again.
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors