[PATCH 1 of 5] context: optimize _parents()

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1448161452 28800
#      Sat Nov 21 19:04:12 2015 -0800
# Node ID a0dca54dfb28f166758d41adc947438850eefa1b
# Parent  f8ab893d8dac5246224f330fafdea35c0bcef313
context: optimize _parents()

This patch avoids some extra attribute lookups and list mutations.

This micro-optimization seems to result in a minor speedup for `hg log`
on my Firefox repo:

before: 25.35s
after:  24.91s
delta:  -0.44s (98% of original)

Not the biggest gain. But every little bit helps.

diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -506,20 +506,21 @@ class changectx(basectx):
         return self._repo.manifest.read(self._changeset[0])
 
     @propertycache
     def _manifestdelta(self):
         return self._repo.manifest.readdelta(self._changeset[0])
 
     @propertycache
     def _parents(self):
-        p = self._repo.changelog.parentrevs(self._rev)
-        if p[1] == nullrev:
-            p = p[:-1]
-        return [changectx(self._repo, x) for x in p]
+        repo = self._repo
+        p1, p2 = repo.changelog.parentrevs(self._rev)
+        if p2 == nullrev:
+            return [changectx(repo, p1)]
+        return [changectx(repo, p1), changectx(repo, p2)]
 
     def changeset(self):
         return self._changeset
     def manifestnode(self):
         return self._changeset[0]
 
     def user(self):
         return self._changeset[1]