context: optimize linkrev adjustment in blockancestors() (issue5538)

We set parent._descendantrev = child.rev() when walking parents in
blockancestors() so that, when linkrev adjustment is perform for these, it
starts from a close descendant instead of possibly topmost introrev. (See
`self._adjustlinkrev(self._descendantrev)` in filectx._changeid().)

This is similar to changeset c82d88dfaf59, which added a "f._changeid"
instruction in annotate() for the same purpose.
However, here, we set _descendantrev explicitly instead of relying on the
'_changeid' cached property being accessed (with effect to set _changeid
attribute) so that, in _parentfilectx() (called from parents()), we go through
`if '_changeid' in vars(self) [...]` branch in which instruction
`fctx._descendantrev = self.rev()` finally appears and does what we want.

With this, we can roughly get a 3x speedup (including in example of issue5538
from mozilla-central repository) on usage of followlines revset (and
equivalent hgweb request).

mercurial/context.py

@@ -1214,6 +1214,10 b' def blockancestors(fctx, fromline, tolin'
                 # introduced in this revision; no need to go futher in this
                 # branch.
                 continue
+            # Set _descendantrev with 'c' (a known descendant) so that, when
+            # _adjustlinkrev is called for 'p', it receives this descendant
+            # (as srcrev) instead possibly topmost introrev.
+            p._descendantrev = c.rev()
             visit[p.linkrev(), p.filenode()] = p, linerange1
         if inrange:
             yield c, linerange2