(bug 33928) make diff title page more relevant

(bug 33928) make diff title page more relevant

Original patch by Nischay Nahata.

Patchset2: amended commit message to be shorter and clearer.
Patchset3: use wfMessage()->text() instead of default parse()
Patchset4: remove unneeded tabs.
Patchset5: added git difference-title to maintenance/language/messages.inc .
Patchset6: moved OutputPage::setPageTitle() calls to DifferenceEngine and added difference-title-multipage message
Patchset7: Fixed an error in messages.inc from the last changeset
Patchset8: Removed the diff page subtitle according to Brion's
           suggestions. Note: I am doing this almost a month since
           committing the original patch. Rebasing proved to be too
           tricky, so I tried to cherry-pick the original commit
           in a new branch. Apparently, Git identified it. If this
           works as planned, it must be a miracle.
Patchset9: removes unneeded changes to message files

Change-Id: I4e6363529a1e77a3f5267ec6871e8dea0354f043