GitDownloadStrategy: avoid unnecessary checkout

By default, git checks out the fetched ref after cloning the repo, but
this is unnecessary as we explicitly check out the desired ref during
staging. This also silences some unnecessarily confusing output.

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

Library/Homebrew/download_strategy.rb

@@ -342,7 +342,7 @@ class GitDownloadStrategy < AbstractDownloadStrategy
 
     unless @clone.exist?
       # Note: first-time checkouts are always done verbosely
-      clone_args = [@@git, 'clone']
+      clone_args = [@@git, 'clone', '--no-checkout']
       clone_args << '--depth' << '1' if support_depth?
 
       case @spec