Use WikiPage::prepareContentForEdit in SpamBlacklistHooks::filterMergedContent

Using WikiPage::prepareContentForEdit instead of
Content::getParserOutput allows us to share the cached parser output
with other hooks that run during the edit process.

Note SpamBlacklistHooks::filterAPIEditBeforeSave already does this.

Bug: 57026
Change-Id: I8c8b293af2842411fd95d3bc21e966a72b2a78b4
Merge "Use WikiPage::prepareContentForEdit in SpamBlacklistHooks::filterMergedContent"