R1:c589af51e8b3

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…