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
