utils/git: quieter git --version.

Let's avoid printing this unless `--debug` (to avoid
`brew bundle check -v` outputting this).

Fixes https://github.com/Homebrew/homebrew-bundle/issues/865

Library/Homebrew/utils/git.rb

@@ -16,7 +16,7 @@ module Utils
     def version
       return @version if defined?(@version)
 
-      stdout, _, status = system_command(git, args: ["--version"], print_stderr: false)
+      stdout, _, status = system_command(git, args: ["--version"], verbose: false, print_stderr: false)
       @version = status.success? ? stdout.chomp[/git version (\d+(?:\.\d+)*)/, 1] : nil
     end