refactors and fixes in guides generation [ci skip]

This commit is not precisely atomic, but the changes have evolved, summary:

* The ENV-based interface has been moved upwards, the generator has now a conventional
  initializer.

* RAILS_VERSION is now assumed to be a Git tag. A blank RAILS_VERSION means edge guides.

* In consequence, the EDGE env variable is gone.

* The "local" version is also gone, the current SHA1 is computed for edge guides.

* Assumes guides are generated from a repo checkout (time ago users could
  generate them from gems.)

* The WARNINGS flag is gone in consequence, you cannot disable warnings.

* The `api_link` Markdown helper is fixed.

* Docs about usage have one single place: rake guides:help.

* Links in guides have been revised.

1 files changed, 23 insertions, 13 deletions

diff --git a/guides/rails_guides.rb b/guides/rails_guides.rb
index 557d23f78c..0f611c8f2b 100644
--- a/guides/rails_guides.rb
+++ b/guides/rails_guides.rb
@@ -1,17 +1,27 @@
-pwd = File.dirname(__FILE__)
-$:.unshift pwd
+$:.unshift __dir__
 
-begin
-  # Guides generation in the Rails repo.
-  as_lib = File.join(pwd, "../activesupport/lib")
-  ap_lib = File.join(pwd, "../actionpack/lib")
+as_lib = File.expand_path("../activesupport/lib", __dir__)
+ap_lib = File.expand_path("../actionpack/lib", __dir__)
+av_lib = File.expand_path("../actionview/lib", __dir__)
 
-  $:.unshift as_lib if File.directory?(as_lib)
-  $:.unshift ap_lib if File.directory?(ap_lib)
-rescue LoadError
-  # Guides generation from gems.
-  gem "actionpack", ">= 3.0"
-end
+$:.unshift as_lib if File.directory?(as_lib)
+$:.unshift ap_lib if File.directory?(ap_lib)
+$:.unshift av_lib if File.directory?(av_lib)
 
 require "rails_guides/generator"
-RailsGuides::Generator.new.generate
+require "active_support/core_ext/object/blank"
+
+env_value = ->(name) { ENV[name].presence }
+env_flag  = ->(name) { "1" == env_value[name] }
+
+version = env_value["RAILS_VERSION"]
+edge    = `git rev-parse HEAD`.strip unless version
+
+RailsGuides::Generator.new(
+  edge:     edge,
+  version:  version,
+  all:      env_flag["ALL"],
+  only:     env_value["ONLY"],
+  kindle:   env_flag["KINDLE"],
+  language: env_value["GUIDES_LANGUAGE"]
+).generate