branchcache: introduce hasbranch()
This will be used to check whether a branch exists or not. This will optimized
in future.
Differential Revision: https://phab.mercurial-scm.org/D6154
branchmap: drop branchcache.setdefault() (API)
All the callers are updated to call setdefault of branchcache.entries
Differential Revision: https://phab.mercurial-scm.org/D6153