Squash merge request compare and commits compare into one page

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Dev: https://dev.gitlab.org/gitlab/gitlabhq/issues/1436

Right now we have 2 different pages for same functionality - comparing commits. We need to make one page from two.

Why?

• it uses different logic that cause different behaviour.
• it causes a lot of duplication in code
• it cause confusion from UI/UX perspective

cc/ @JobV @dzaporozhets @sytses