localrepo: add ignoremissing parameter to branchtip
Previously, in the namespaces api, the only caller of branchtip was singlenode
which happened to raise the same exception that branchtip raised: KeyError.
This is a minor change but will allow upcoming patches to use repo.branchtip to
not raise an exception if a branch doesn't exist. After that, it will be
possible for extensions to use the namespace api in a stable way.
(binary:application/octet-stream)