mw.ViewPageTarget: Fix incorrect retention of the wrong oldid

* Only pass the oldid to the API from #load if we restoring from
  oldid in the url. Otherwise load the latest version.
* Setting 'restoring' from mw.Target instead of mw.ViewPageTarget
  so that we don't rely on mw.ViewPageTarget in mw.Target#load.
* Fix the API to not require 'oldid' to be passed.
* Fix the API to actually return the 'newrevid' property. It
  was doing a no-op on a $result that is never used due to the
  same variable being overwritten with the result of parseWikitext.
* Moved updating of wgCurRevisionId to mw.ViewPageTarget as it
  belongs there (possible future inline editors probably act
  on a different page than the main one). Also made it only
  update if it isn't undefined, so that a null edit doesn't
  result in wgCurRevisionId being unset.

Bug: 49943
Bug: 50441
Change-Id: I221e5038f95eadf6d87013e80f12394f0376a293
Merge "mw.ViewPageTarget: Fix incorrect retention of the wrong oldid"