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.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown