Fix reverse: flag keep

[fakeboboliu]

This fixes issue caused by 6ac80db.

reverse exits if we have a package that won't build with empty build constraints.

[luantak]

Can you explain exactly what this fixes

[fakeboboliu]

Can you explain exactly what this fixes

consider a main.go:

//go:build sometag

func main() {
    return
}

garble reverse -tags=sometag . will shows build constraints exclude all Go files.

[luantak]

Can you add a regression test for this to:

https://github.com/burrowers/garble/blob/master/testdata%2Fscript%2Freverse.txtar

?

How to test:
https://github.com/burrowers/garble/blob/master/CONTRIBUTING.md#testing

[fakeboboliu]

Can you add a regression test for this to:

https://github.com/burrowers/garble/blob/master/testdata%2Fscript%2Freverse.txtar

done.

before:

> go test -run Script/reverse
 ......
            > exec ./tag.bin
            [stderr]
            main.NMSpPYY2n3
            > cp stderr tag.stderr
            > ! grep "main.CallerFuncName" tag.stderr
            > stdin tag.stderr
            > exec garble reverse -tags sometag ./case/tag
            [stderr]
            build constraints exclude all Go files in $WORK\case\tag
            [exit status 1]
            FAIL: testdata\script\reverse.txtar:54: unexpected command failure
 ......

after:

> go test -run Script/reverse
PASS
ok      mvdan.cc/garble 11.716s

[mvdan]

Thanks!

[mvdan]

Pretty embarassing that I didn't notice the mistake in my "simplification" refactor.