Present the commit title on merge SHA tooltip on MR widget

Currently we don't present a tooltip on the following merge commit SHA link:

That's mainly because we don't persist this information on DB and would need to request Gitaly for the commit title information (and MergeRequestsController#show.json endpoint is one of the most requested endpoints at the moment).

To make it consistent with Note's UI, we should somehow cache and present it on the MR widget UI as well.