Clone only the tag needed to generate the docs

Before, for each released version, we were cloning the entire Rails
repository in a new directory, then checking out the tag we wanted.
This is inefficient in both time and space.

Our server is full of copies of the Rails repository, one per released
version, and each copy contains the full history of the repository,
using gigabytes of disk space.

To save in both time and space, we can clone only the specific tag we
need, using git's `--branch` and `--depth 1` options.

Author: rafaelfranca

lib/git_manager.rb

@@ -39,11 +39,7 @@ def update_main
   def checkout(tag)
     Dir.chdir(basedir) do
       log "checking out tag #{tag}"
-      log_and_system "git clone -q #{remote_rails_url} #{tag}"
-
-      Dir.chdir(tag) do
-        log_and_system "git checkout -q #{tag}"
-      end
+      log_and_system "git -c advice.detachedHead=false clone -q --depth 1 --single-branch --branch #{tag} #{remote_rails_url} #{tag}"
     end
   end