gh-64 added exclude argument in checksums

[ethicaladitya]

Added argument "--excluded" to the wp core verify-checksums to rule out any false positives when we have extra files in the core.

Related: https://github.com/wp-cli/checksum-command/issues/64 https://github.com/wp-cli/wp-cli/issues/5955

[mrsdizzie]

~I think this would need something more specific than strpos because that is for matching substrings and these should be exact matches. For example, if you exclude config.php it should ignore a top level config.php but still flag wp-admin/config.php as an unknown file. There should be another behat test to make sure it doesn't do that.~

~It also mentions excluding directories, but I'm not sure that is necessary since the command already ignores non-core directories both at the top level and within core folders as far as I can see. So it shouldn't be flagging anything inside of an unknown directory already. If there is a specific example of it doing that, or why that would be needed, there should be a test for that as well.~

Edit: now I see a similar PR with that approach was already opened around the same time as this: https://github.com/wp-cli/checksum-command/pull/123

This one should probably be closed in favor of that

[swissspidy]

Actually, it looks like #123 covers both my points already, so I am closing this PR in favor of it.