cmd/go/internal/modfetch: simplify a redundant condition
In gitRepo.stat, we were checking ref != "" twice,
which confused me during casual reading because it made it seem like
the string could be empty when it actually never is.
For #56881.
Change-Id: Ib83303a149ea771399cb55fedd5dfa02ad411ff0
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/549855
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Bryan Mills <bcmills@google.com> Reviewed-by: Michael Matloob <matloob@golang.org>