revlog: add a method to tells whether rev is stored as a snapshot
For now we only have one type of snapshot: full snapshot versus nullrev.
However we are looking into adding intermediate snapshot where a large diff
against another snapshot is performed instead of storing a full new text.
The conditional is a bit strange and is done in order to help readability of a
some later changesets.
#require docutils gettext
$ $TESTDIR/check-gendoc zh_CN
checking for parse errors