cmd/go/internal/get: remove '--' separator from 'git ls-remote' command

'git ls-remote' started recognizing the '--' separator at some point
after 2.7.4, but git defaults to version 2.7.4 on Ubuntu 16.04 LTS,
which remains supported by Ubuntu until April 2021.

We added '--' tokens to most VCS commands as a defensive measure in
CL 181237, but it isn't strictly necessary here because the 'scheme'
argument to our template is chosen from a predefined list: we can
safely drop it to retain compatibility.

Fixes #33836
Updates #26746

Change-Id: Ibb53366b95f8029b587e0b7646a439330d759ac7
Reviewed-on: https://go-review.googlesource.com/c/go/+/191978
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

src/cmd/go/internal/get/vcs.go

@@ -164,8 +164,14 @@ var vcsGit = &vcsCmd{
 	// See golang.org/issue/9032.
 	tagSyncDefault: []string{"submodule update --init --recursive"},
 
-	scheme:     []string{"git", "https", "http", "git+ssh", "ssh"},
-	pingCmd:    "ls-remote -- {scheme}://{repo}",
+	scheme: []string{"git", "https", "http", "git+ssh", "ssh"},
+
+	// Leave out the '--' separator in the ls-remote command: git 2.7.4 does not
+	// support such a separator for that command, and this use should be safe
+	// without it because the {scheme} value comes from the predefined list above.
+	// See golang.org/issue/33836.
+	pingCmd: "ls-remote {scheme}://{repo}",
+
 	remoteRepo: gitRemoteRepo,
 }