Previously, when adding blocks for the snippets' content, the
operation macro would call Asciidoctor to load the content with
attributes that only included the projectdir. This resulted in a
failure in environments that did not set the projectdir and instead
relied on another attribute, such as gradle-projectdir as used by
the latest versions of the Asciidoctor Gradle plugin.
This commit updates the macro to reuse all of the main document's
attributes when loading the snippets' content, thereby ensuring that
any attributes available in the main document are also available
during snipppet processing.
Closes gh-624
619847582a