Put a limit on the number of files showing in herald emails

Summary:
Sometimes a commit can be huge (like a branch cut in FB www which could have more than half a million files touched). It will generate some emails with size more than 30M, and it will take quite a while to just sort the files and to send out.
Put a hard limit here to avoid such cases. Probably only matters for FB right now, but still even for a small repo with several thousand files, it is a waste to send them all out. Not sure if there is any cleaner way to do it though.

Test Plan: Tried it in FB installtion.

Reviewers: lifeihuang, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8889