git-flow: fix broken HEAD build

Because the subordinate formula GitFlowCompletion defines 'head', its constructor
requires a 'name' argument when --HEAD is used.  The 'name' attribute is required
to create the Git repo for the formula.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/git-flow.rb

@@ -20,7 +20,7 @@ class GitFlow < Formula
   def install
     system "make", "prefix=#{prefix}", "install"
 
-    GitFlowCompletion.new.brew do
+    GitFlowCompletion.new('git-flow-completion').brew do
       (prefix+'etc/bash_completion.d').install "git-flow-completion.bash"
       (share+'zsh/site-functions').install "git-flow-completion.zsh"
     end