legit: install completion files and add head

Closes https://github.com/kennethreitz/legit/issues/47

Closes Homebrew/homebrew#43402.
Closes Homebrew/homebrew#43406.

Signed-off-by: Xu Cheng <xucheng@me.com>

Formula/legit.rb

@@ -3,6 +3,8 @@ class Legit < Formula
   homepage "http://www.git-legit.org/"
   url "https://github.com/kennethreitz/legit/archive/v0.2.0.tar.gz"
   sha256 "dce86a16d9c95e2a7d93be75f1fc17c67d3cd2a137819fa498e179bf21daf39e"
+  head "https://github.com/kennethreitz/legit.git", :branch => "develop"
+  revision 1
 
   bottle do
     cellar :any
@@ -54,6 +56,10 @@ class Legit < Formula
 
     bin.install Dir["#{libexec}/bin/*"]
     bin.env_script_all_files(libexec/"bin", :PYTHONPATH => ENV["PYTHONPATH"])
+
+    bash_completion.install "extra/bash-completion/legit"
+    zsh_completion.install "extra/zsh-completion/_legit"
+    man1.install "extra/man/legit.1" if build.head?
   end
 
   test do