Make remote/HEAD resolve to a commit id

[thinkerbot]

The remote/HEAD references currently do not resolve to a commit id because the HEAD file contains a ref rather than a sha:

% irb -r grit
>> puts Grit::Repo.new('.').remotes.map {|remote| "#{remote.name}: #{remote.commit.id}" }.join("\n")
origin/HEAD: ref:
origin/master: 0bd0c5ff037c7368b2b1827fadc66741d1ed14ed
=> nil
>> exit

% cat .git/refs/remotes/origin/HEAD 
ref: refs/remotes/origin/master

A patch is here: bahuvrihi@31bf65df77e6d9b3df75d9e314e2e9c57a48384d

[bkeepers]

Grit is no longer maintained. See #183 and check out libgit2/rugged.