branchmap: micro-optimize branchinfo

changelogrevision() is supposed to be used if not all data of
changelog.read is used. This is the case here as only the extra field is
used. This also improves extensibility as at least hgext.git doesn't
implement changelog.read.

Differential Revision: https://phab.mercurial-scm.org/D9626

mercurial/changelog.py

@@ -601,7 +601,7 b' class changelog(revlog.revlog):'
 
         This function exists because creating a changectx object
         just to access this is costly."""
-        extra = self.read(rev)[5]
+        extra = self.changelogrevision(rev).extra
         return encoding.tolocal(extra.get(b"branch")), b'close' in extra
 
     def _nodeduplicatecallback(self, transaction, node):