Avoid loading the wiki page content when listing pages
There are situations where we request the wiki page content but we don't need it at all. For example when we load the sidebar or the pages page. In this case, we just need to load the page title, but we load the whole page. If we have several "big" wiki pages the performance will be affected.
This change will reduce the wiki page load time, the number of Gitaly RPC calls and the number of IO operations.
/cc @jramsay
Edited by Francisco Javier López (ex-Gitlab)