context: fast path linkrev adjustement in trivial case

If the search starts from the linkrev, there is nothing to adjust.

mercurial/context.py

@@ -734,6 +734,8 b' class basefilectx(object):'
         mfl = repo.manifestlog
         # fetch the linkrev
         lkr = self.linkrev()
+        if srcrev == lkr:
+            return lkr
         # hack to reuse ancestor computation when searching for renames
         memberanc = getattr(self, '_ancestrycontext', None)
         iteranc = None