cmd/go/internal/modfetch: set protocol.version=2 for shallow git fetches

This works around an apparent bug in the Git HTTP backend, introduced
in Git 2.21, that causes responses for the version 1 protocol to
provide incomplete tags.

For Git commands older than 2.18, this configuration flag is ignored.
(Note that Git 2.29 and above already use protocol version 2 by
default.)

Fixes #56881.

Change-Id: I9b241cfb604e5f633ca6a5d799df6706246684a7
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/556358
Run-TryBot: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/modfetch/codehost/git.go

@@ -550,7 +550,11 @@ func (r *gitRepo) stat(ctx context.Context, rev string) (info *RevInfo, err erro
 		if err != nil {
 			return nil, err
 		}
-		_, err = Run(ctx, r.dir, "git", "fetch", "-f", "--depth=1", r.remote, refspec)
+		// We explicitly set protocol.version=2 for this command to work around
+		// an apparent Git bug introduced in Git 2.21 (commit 61c771),
+		// which causes the handler for protocol version 1 to sometimes miss
+		// tags that point to the requested commit (see https://go.dev/issue/56881).
+		_, err = Run(ctx, r.dir, "git", "fetch", "-f", "-c", "protocol.version=2", "--depth=1", r.remote, refspec)
 		release()
 
 		if err == nil {