[PATCH 1 of 7 V4] revlog: clarify the non sparse-revlog case in `issnapshot`

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1546182882 -3600
#      Sun Dec 30 16:14:42 2018 +0100
# Node ID 88025ac40a127963ea8a186ae5e81f64002db39a
# Parent  88d813cd9acd56490fdac2c0b8e66c5231c63d88
# EXP-Topic sparse-revlog
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 88025ac40a12
revlog: clarify the non sparse-revlog case in `issnapshot`

If we are not using sparse-revlog, there is only one type of snapshot, the full
one.

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1538,6 +1538,8 @@ class revlog(object):
         deltap = self.deltaparent(rev)
         if deltap == nullrev:
             return True
+        elif not self._sparserevlog:
+            return False
         p1, p2 = self.parentrevs(rev)
         if deltap in (p1, p2):
             return False