cmd/go: on malformed "go mod edit -go", suggest current version of go, not hardcoded 1.12

[mark-rushakoff]

What version of Go are you using (go version)?

$ go version
go version devel +0f897f916a Tue May 28 02:52:39 2019 +0000 darwin/amd64

Does this issue reproduce with the latest release?

n/a, go mod edit suggests -go 1.12 on go1.12.5.

What did you do?

I wasn't sure if there was a way to go mod edit to put the go 1.12 directive in the "right" place in 1.12, prior to #31960 on tip, so I ran with a garbage argument to -go. This command has the same output in 1.12 and tip:

$  go mod edit -go=x
go mod: invalid -go option; expecting something like "-go 1.12"

What did you expect to see?

I expected the output to suggest -go 1.13 when running with tip.

What did you see instead?

It suggested -go 1.12, which seems odd given that I am running what is going to be 1.13.

I suppose there's an argument to suggest 1.12 as a "downgrading" case when go.mod already provides go 1.13, but the argument to use -go 1.13 to upgrade from a go 1.12 in go.mod seems just as valid.

[bcmills]

The “something like” phrase in that error message is critical. 🙂

[bcmills]

Fixed in CL 321069.