Summary:
Ultimate goals of the coverage report are:
* Report the coverage for all files (done in this diff)
* Report the coverage for recently updated files (not fully finished)
* Report is available in html form (done in this diff, but need some extra work to integrate it in Jenkin)
Task link: https://our.intern.facebook.com/intern/tasks/?s=1154818042&t=2604914
Test Plan:
Ran: coverage/coverage_test.sh
The sample output can be found here: https://phabricator.fb.com/P2433892
Reviewers: dhruba, emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D11943
