fix!: Include -- in passthrough args

[boblail]

Given a grammar like this:

Issue

var cli struct {
        Args []string `arg:"" optional:"" passthrough:""`
}

If Kong parses cli foo -- bar, it will populate Args with []string{"foo", "--", "bar"} (including "--"). However, if Kong parses cli -- foo bar, will populate Args with []string{"foo", "bar"} (leaving off "--").

This differs from the behavior of a passthrough Command, where "--" is included with the args in both cases.

Proposal

There are 3 places where c.endParsing() is called

1. When node.Passthrough is true:

   kong/context.go

   Lines 366 to 368 in 5f9c5cc

   	if node.Passthrough {
   	c.endParsing()
   	}

2. When arg.Passthrough is true:

   kong/context.go

   Lines 451 to 453 in 5f9c5cc

   	if arg.Passthrough {
   	c.endParsing()
   	}

3. When "--" is encountered:

   kong/context.go

   Lines 384 to 387 in 5f9c5cc

   	// Indicates end of parsing. All remaining arguments are treated as positional arguments only.
   	case v == "--":
   	c.scan.Pop()
   	c.endParsing()

The first two do not also pop any tokens. The third one does.

This commit makes c.scan.Pop() conditional, skipping it when the next positional argument is passthrough.

Breaking Change

I believe this will cause Kong to behave a little more consistently — and from my perspective, -- is relevant for args intended to be passed through! — but it will change the behavior of existing projects that use arg:"" passthrough:"".

[alecthomas]

This was actually intentional for compatibility with the previous behaviour of passthrough, but I think it's okay to change it. I've been meaning to cut a 1.0 release, and this is a good motivation.

[alecthomas]

If you find any other consistencies, now would be the time.

[alecthomas]

Released in v1.2.0

[AlekSi]

So technically, it was a breaking change in a minor release (1.1 -> 1.2). Having a changelog with a list of breaking changes would be nice.

[alecthomas]

"Technically", yes, but that was mainly because I forgot to include it in the 1.0 release. It was added very shortly after, so I'm okay with it.

This is the only breaking change.

[alecthomas]

I added a note to the README about the 1.0 release.