rake/preindex: return "content" from expand!
rake/preindex: return "content" from expand!

Otherwise we not only fail to fix the recursive case in
1288eb5 (recursively include files in asciidoc, 2016-04-26),
but we break the non-recursive one, too.

Mokolea/git-scm.com,Mokolea/git-scm.com,Mokolea/git-scm.com,mosoft521/gitscm-next,jnavila/gitscm-next,mosoft521/gitscm-next,git/git-scm.com,mosoft521/gitscm-next,jasonlong/git-scm.com,jnavila/gitscm-next,jasonlong/git-scm.com,git/git-scm.com,jasonlong/git-scm.com,mosoft521/gitscm-next,jnavila/gitscm-next,git/git-scm.com,jnavila/gitscm-next,jasonlong/git-scm.com,git/git-scm.com,Mokolea/git-scm.com