Optimize compute case where the content isn't used

Created by: rvantonder

Some queries might use something like output((.|\n)* -> $repo) to say "match all content" for commit, diff, or file results. We don't need to run the (.|\n)* regex if the right hand side doesn't use the content result (i.e., regexp capture group) and can optimize to emit only the non-content "metadata" for the result.

/cc @benvenker @lguychard