add "update" mode to Diffusion coverage Conduit

Summary:
This diff adds a new mode argument to the Diffusion Conduit API with two options:

• "overwrite": the default, maintains the current behavior of deleting all coverage in the specified branch before uploading the new coverage
• "update": does not delete old coverage, but will overwrite previous coverage information if it's for the same file and commit

DiffusionRequest::loadCoverage already loads a file's coverage from the
latest available commit, so uploading coverage for different files in different
commits with "update" will result in seeing the latest uploaded coverage in
Diffusion.

Test Plan: manual local verification

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14428