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.