When the command searches args to find the arg matching a
particular subcommand name, it needs to ignore flag values,
as it is possible that the value for a flag might match
the name of the sub command.
This change improves argsMinusFirstX() to ignore flag values
when it searches for the X to exclude from the result.
This fixes a bug where a child flag that shadows (has the same
name as) a parent persistent flag would not be shown in the
child command's help output and the parent flag would be shown
instead under the global flags section.
This change makes the help output consistent with the
observed behavior during execution, where the child flag is
the one that is actually used.