Allow commit search / filtering in the commits tab of a merge request

Description

Especially during CE->EE merges, there is a use case for checking whether a given commit is present in a merge request. So I might navigate to, e.g., https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3007/commits and scroll down the list or use my browser's search functionality to find the commit. For larger MRs, this tab can be extremely slow to load and I might go straight to the branch page instead: https://gitlab.com/gitlab-org/gitlab-ee/commits/ce-to-ee

Proposal

Add built-in search / filtering for the list of commits we present. Both by message and author would be best. Paginating the list of commits (while still allowing filtering across the whole range) would also be useful for larger MRs.

Links / references

Screenshot_from_2017-09-25_12-13-44 Screenshot_from_2017-09-25_12-14-28 Screenshot_from_2017-09-25_12-14-44

Documentation blurb

Overview

What is it? Why should someone use this feature? What is the underlying (business) problem? How do you use this feature?

Use cases

Who is this for? Provide one or more use cases.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

/cc @victorwu @smcgivern