commands: add more robust support for 'hg log -b' (issue2078) Fixes issue2078 and adds tests to cover various 'hg log -b' uses. This change adds a localrepo.lookupbranch(key, remote=None) function. This will look up the branch of the revision with the given key. The algorithm works like this: * If a remote repo is given and KEY is the name of a branch in that repo, return KEY. * If no remote repo is given and KEY is the name of a branch in the local repo object, return KEY. * Otherwise look up the revision with the identifier KEY in the local repo and return its branch. This change also makes 'hg log -b' use this new functionality and adds a few tests for it.