error: unknown command selector "--help"
usage: go tool covdata [command]

Commands are:

textfmt     convert coverage data to textual format
percent     output total percentage of statements covered
pkglist     output list of package import paths
func        output coverage profile information for each function
merge       merge data files together
subtract    subtract one set of data files from another set
intersect   generate intersection of two sets of data files
debugdump   dump data in human-readable format for debugging purposes

For help on a specific subcommand, try:

go tool covdata <cmd> -help