x/text: avoid double execution of lookup in lookupAndFormat()

[fgm]

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

$ go version
go version go1.11.2 linux/amd64

Does this issue reproduce with the latest release?

• This is the latest release.
• code is identical in the master branch

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/fgm/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/fgm/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
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-build766242374=/tmp/go-build -gno-record-gcc-switches"

What did you do?

• Prepare a simple app with exported strings using //go:generate gotext -srclang=en update -out=catalog/catalog.go -lang=fr
• Invoke p.Printf("Hello, world") and debug what happens, until stepping into message.lookupAndFormat() until this fragment:

	if p.catContext.Execute(id) == catalog.ErrNotFound {
		if p.catContext.Execute(msg) == catalog.ErrNotFound {
			p.Render(msg)
			return
		}

What did you expect to see?

• since in most cases id == msg, skip the second execution of p.catContext(msg) if the first execution returned catalog.ErrNotFound

What did you see instead?

• if p.catContext.Execute(id) returned catalog.ErrNotFound, code performs the same work a second time as p.catContext.Execute(msg)
• which could be avoided by checking whether id == msg, maybe like:

	if p.catContext.Execute(id) == catalog.ErrNotFound {
		if id == msg || p.catContext.Execute(msg) == catalog.ErrNotFound {
			p.Render(msg)
			return
		}
	}

[bcmills]

CC: @mpvl

[fgm]

Still present in v0.3.2 (12/2018) and current master HEAD. I could prepare a PR if you so wish.