Store empty (default) branch in branch cache, too.
Operations on this branch don't work otherwise.
Reading branches.cache had to be adjusted to allow an empty label.
Adjusted reading of the cache tip for symmetry, no functional change here.
changeset: 0:9c2cf2b35aa7
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: add foo
changeset: 1:3756a9556b89
tag: tip
user: test
date: Mon Jan 12 13:46:41 1970 +0000
summary: change foo
2 revisions:
9c2cf2b35aa7
3756a9556b89