Use deep_merge when combining CI content

When combining GitLab CI YAML content, use deep_merge instead of merge
to combine external files and the inline content. This merges the
external files and inline content recursively, providing more options
for customizing jobs loaded from external files.

ee/lib/gitlab/ci/external/processor.rb

@@ -15,7 +15,7 @@ module Gitlab
 
           external_files.each do |external_file|
             validate_external_file(external_file)
-            @content.merge!(content_of(external_file))
+            @content.deep_merge!(content_of(external_file))
           end
 
           append_inline_content
@@ -37,7 +37,7 @@ module Gitlab
         end
 
         def append_inline_content
-          @content.merge!(@values)
+          @content.deep_merge!(@values)
         end
 
         def remove_include_keyword