changelog: add a `branch` method, bypassing changectx

The only way to access the branch of a changeset is currently to
create a changectx object and access its `branch()` method. Creating
a new Python object is costly and has a huge impact on code doing
heavy access to `branch()` (like branchmap).

This change introduces a new method on changelog that allows direct
access to the branch of a revision. See the next changeset for impact.

mercurial/changelog.py

@@ -337,3 +337,10 b' class changelog(revlog.revlog):'
         l = [hex(manifest), user, parseddate] + sorted(files) + ["", desc]
         text = "\n".join(l)
         return self.addrevision(text, transaction, len(self), p1, p2)
+
+    def branch(self, rev):
+        """return the branch of a revision
+
+	This function exists because creating a changectx object
+	just to access this is costly."""
+        return encoding.tolocal(self.read(rev)[5].get("branch"))