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
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..