GitDownloadStrategy: don't assume --single-branch is available

git-clone's --single-branch is too new, and Xcode 4.3's stock git
doesn't have it. Since we don't require git from Homebrew, don't use it.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/download_strategy.rb

@@ -349,7 +349,6 @@ class GitDownloadStrategy < AbstractDownloadStrategy
     unless @clone.exist?
       # Note: first-time checkouts are always done verbosely
       clone_args = %w[git clone]
-      clone_args << '--single-branch'
       clone_args << '--depth' << '1' if support_depth?
 
       case @spec