Currently, doing `brew update-reset homebrew/core` does nothing (not
even return an error). If you want to `update-reset` a given tap, you
must do (the equivalent of)

    brew update-reset "$(brew --repository owner/tap_name)"

This isn't very intuitive, so let's do a bit more work in argument
parsing so that the user can just pass a tap name instead of a path to a
tap.

Passing a path to a tap is also still supported.

This reverts commit ac7dbc0.

This is based on feedback from Homebrew#14667.

This command was being called with the wrong relative path, so it
silently did nothing. Now that `update-reset` errors out from invalid
arguments, we know that running `update-reset` here is not needed.