MCR: rename $baseRevId paramter to match actual semantics.

MCR: rename $baseRevId paramter to match actual semantics.

The $baseRevId in WikiPage::doEditContent is used only to indicate what
revision an edit reverted to. It is not used to indicate the actual base
revision of an edit in any sense. Specifically, EditPage never sets it.

So, this change renames the parameter to $originalRevId to match $undidRevId.
It also renames PageUpdater::setBaseRevisionId to setOriginalRevisionId.
Further, this introduces a paramter to PageUpdater::hasEditConflict():

Before this change, PageUpdater::hasEditConflict() was based on the
revision set via PageUpdater::setBaseRevisionId(), assuming the semantics
of "base revision" used by EditPage. However, this is NOT how the $baseRevId
parameter in WikiPage works.

Bug: T197685
Change-Id: Ib78257d4d6ee7c4ec093d5706904c599b02c73e0