deltaparent(): don't return nullrev for a revision containing a full snapshot
this allows us to simplify manifest.readdelta and revlog.revdiff
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ echo '[extensions]' >> $HGRCPATH
$ echo "gpg =" >> $HGRCPATH
$ echo "hgext.gpg =" >> $HGRCPATH
$ echo "badext = $abspath" >> $HGRCPATH
$ echo "badext2 =" >> $HGRCPATH
$ hg -q help help
\*\*\* failed to import extension badext from .*/badext.py: bit bucket overflow
\*\*\* failed to import extension badext2: No module named badext2
hg help [TOPIC]
show help for a given topic or a help overview