Limit amount of chart data passed to browser

Summary:
Firefox has a limit of ~500,000 elements which can be passed in literal array.
This amount of data is meaningless anyway because even Retina displays don't have such resolution.

Limit the amount of data to mitigate browser limitations and also reduce the page size.
Ensure that first and last element is passed.

I considered also reducing the granularity to days but I want new repositories to have nice precise graph.

Test Plan: Displayed the chart in Firefox.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3134