revlog: finer computation of "issnapshot"

If the parent had an empty diff, we skip of it to compute a diff against the parent base. This create shorter and simpler chain.

However these case could be wrongly detected as snapshot. So we improve the code doing this detection.

In practice nobody care as when tried on a copy of mozilla-try and we got the same number of snapshot (1315) in both case.

Performance where equivalent.