branchmap: make branchcache responsible for reading
Encapsulate reading in a classmethod, to make it clear what kind of object is
being handled.
This is part of a stack of refactoring changes to help performance improvements
down the line.
Differential Revision: https://phab.mercurial-scm.org/D5635
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors