Restrict the conditions under which we try to load initial page

Restrict the conditions under which we try to load initial page
contents from pgsrc.

Now we only load pgsrc iff the user is attempt to browse the HomePage.

This allows some hope of fixing things if the configured pgsrc, e.g.
doesn't include a HomePage.  (Without this patch, one gets stuck in a
loop --- every attempt to do anything with the wiki results in a
load of pgsrc.)

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@486 96ab9672-09ca-45d6-a79d-3d69d39ca109