Returns the coverage report for a given commit.
By default that commit is the head of the default branch but can also be specified explictily by:
sha - return report for the commit with the given SHA
branch - return report for the head commit of the branch with the given name
The report can be optionally filtered by specifying:
path - only show report info for pathnames that start with this value
flag - only show report info that applies to the specified flag name