git-subtree: add `--build-docs` option

The man page for this is really essential. If the user choose not to
build the man page, simply copy the original `asciidoc` file to #{doc}.

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

Formula/git-subtree.rb

@@ -3,12 +3,30 @@ require 'formula'
 class GitSubtree <Formula
   homepage 'http://github.com/apenwarr/git-subtree'
   head 'git://github.com/apenwarr/git-subtree.git',
-        :tag => 'c00d1d11688dc02f066196ed18783effdb7767ab'
+        :tag => 'v0.3'
+
+  def options
+    [
+      ['--build-docs', "Build man pages using asciidoc and xmlto"]
+    ]
+  end
+
+  if ARGV.include? '--build-docs'
+    # these are needed to build man pages
+    depends_on 'asciidoc'
+    depends_on 'xmlto'
+  end
 
   # Not depending on git because people might have it
   # installed through another means
 
   def install
+    if ARGV.include? '--build-docs'
+      system "make doc"
+      man1.install "git-subtree.1"
+    else
+      doc.install "git-subtree.txt"
+    end
     bin.install "git-subtree.sh" => "git-subtree"
   end
 end