context: remove assumptions about manifest creation during _buildstatus

Previously we called self.manifest() in some cases to preload the
first manifest. This relied on the assumption that the later
_manifestmatches() call did not duplicate any work the original
self.manifest() call did. Let's remove that assumption, since it bit
me during my refactors of this area and is easy to remove.

mercurial/context.py

@@ -117,10 +117,12 b' class basectx(object):'
         # 1000 and cache it so that when you read 1001, we just need to apply a
         # delta to what's in the cache. So that's one full reconstruction + one
         # delta application.
+        mf2 = None
         if self.rev() is not None and self.rev() < other.rev():
-            self.manifest()
+            mf2 = self._manifestmatches(match, s)
         mf1 = other._manifestmatches(match, s)
-        mf2 = self._manifestmatches(match, s)
+        if mf2 is None:
+            mf2 = self._manifestmatches(match, s)
 
         modified, added = [], []
         removed = []