Enable passing env var paths to cli -m option

Currently, if you add the --template/-m option to a railsrc file, you
lose the ability to define the path to your rails template using an
environment variable, which you have when passing that option from the
command line.

This patch adds that ability by shelling out to interpret the passed
path.

railties/lib/rails/generators/app_base.rb

@@ -148,7 +148,7 @@ module Rails
           when /^https?:\/\//
             options[:template]
           when String
-            File.expand_path(options[:template], Dir.pwd)
+            File.expand_path(`echo #{options[:template]}`.strip)
           else
             options[:template]
           end

railties/test/generators/app_generator_test.rb

@@ -720,6 +720,25 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_match(/It works from file!/, run_generator([destination_root, "-m", "lib/template.rb"]))
   end
 
+  def test_template_from_url
+    url = "https://raw.githubusercontent.com/rails/rails/f95c0b7e96/railties/test/fixtures/lib/template.rb"
+    FileUtils.cd(Rails.root)
+    assert_match(/It works from file!/, run_generator([destination_root, "-m", url]))
+  end
+
+  def test_template_from_abs_path
+    absolute_path = File.expand_path(Rails.root, "fixtures")
+    FileUtils.cd(Rails.root)
+    assert_match(/It works from file!/, run_generator([destination_root, "-m", "#{absolute_path}/lib/template.rb"]))
+  end
+
+  def test_template_from_env_var_path
+    ENV["FIXTURES_HOME"] = File.expand_path(Rails.root, "fixtures")
+    FileUtils.cd(Rails.root)
+    assert_match(/It works from file!/, run_generator([destination_root, "-m", "$FIXTURES_HOME/lib/template.rb"]))
+    ENV.delete("FIXTURES_HOME")
+  end
+
   def test_usage_read_from_file
     assert_called(File, :read, returns: "USAGE FROM FILE") do
       assert_equal "USAGE FROM FILE", Rails::Generators::AppGenerator.desc