Allow access to revision deleted wikitext on action=edit

EditPage.php has a Revision::getContent() without specifiy the
visiblity, that will give a empty textbox for revision deleted revisions
of a page, when editing this with the unhide=1 param.
Adding Revision::FOR_THIS_USER to allow admins or oversighter to see the
wikitext of such a revision.

Change-Id: Ie010346b613d96eacee6e33be0ed141f7061b709
Merge "Allow access to revision deleted wikitext on action=edit"