branchcache: move loading of branch names and nodes into it's own function
This will help me in implementing lazy loading of the branchcache in upcoming
patches.
Differential Revision: https://phab.mercurial-scm.org/D6023
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[255]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate