Add github fetch <user> <branch> command.

This will create a remote for <user>, fetch the branch into refs/remotes/<user>/<branch> and then create a local branch from that. This is good for getting an exact copy of the branch (without merging it into another branch) for local testing or inspection.

commands/commands.rb

@@ -61,6 +61,17 @@
   end
 end
 
+desc "Fetch from a remote to a local branch."
+command :fetch do |user, branch|
+  die "Specify a user to pull from" if user.nil?
+  user, branch = user.split("/", 2) if branch.nil?
+  branch ||= 'master'
+  GitHub.invoke(:track, user) unless helper.tracking?(user)
+
+  git "fetch #{user} #{branch}:refs/remotes/#{user}/#{branch}"
+  git_exec "checkout -b #{user}/#{branch} refs/remotes/#{user}/#{branch}"
+end
+
 desc "Pull from a remote."
 flags :merge => "Automatically merge remote's changes into your master."
 command :pull do |user, branch|