[PATCH 4 of 8] revlog: more efficient implementation for issnapshot

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1545040296 -3600
#      Mon Dec 17 10:51:36 2018 +0100
# Node ID 9fe9cc49f235311269fd957c49898396ed7bdfc0
# Parent  4cafb262b243b02c217fb538165e354f77ce0fd8
# EXP-Topic sparse-revlog-corner-cases
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 9fe9cc49f235
revlog: more efficient implementation for issnapshot

We avoid multiple method calls and tuple creation, this provides a significant
speedup in some case:

example affected manifest write
before: 0.815520s
after:  0.487767s (-40%)

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1535,11 +1535,17 @@ class revlog(object):
         """
         if rev == nullrev:
             return True
-        deltap = self.deltaparent(rev)
+        entry = self.index[rev]
+        deltap = entry[3]
+        if deltap == rev:
+            return True
+        if not self._generaldelta:
+            return False
         if deltap == nullrev:
             return True
-        p1, p2 = self.parentrevs(rev)
-        if deltap in (p1, p2):
+        p1 = entry[5]
+        p2 = entry[6]
+        if deltap == p1 or deltap == p2:
             return False
         return self.issnapshot(deltap)