x/exp/cmd/gorelease: support for using @latest as a version

[myitcv]

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

$ go version
go version devel +151ccd4bdb Mon Feb 24 02:36:05 2020 +0000 linux/amd64
$ git rev-parse HEAD
7c80518d1cc79ffde9e571fe4fd281f321d36200

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOENV="/home/myitcv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/myitcv/gos"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build193807698=/tmp/go-build -gno-record-gcc-switches"

What did you do?

$ gorelease -base=latest

What did you expect to see?

A comparison with the latest semver version, resolved via proxy.golang.org

What did you see instead?

usage: gorelease -base=version [-version=version]
base version "latest" is not a canonical semantic version
For more information, run go doc golang.org/x/exp/cmd/gorelease

cc @jayconrod

[bcmills]

That reminds me, I owe @jayconrod a code review on CL 216078.

I think latest makes sense as the default value inferred for -base, although it probably should be supported as an explicit value as well.

[gopherbot]

Change https://golang.org/cl/236598 mentions this issue: cmd/gorelease: accept version queries for -base