Revert "cmd/go: allow omitted user name in git ssh repo syntax"

This caused #13657.
Reverting fixes #13657.

I was trying to be helpful by fixing #12313,
but I don't need the fix myself.
Will leave for someone with more motivation.

This reverts commit 3e9f0636.

Change-Id: Ifc78a6196f23e0f58e3b9ad7340e207a2d5de0a6
Reviewed-on: https://go-review.googlesource.com/17977Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/vcs.go

@@ -147,7 +147,7 @@ var vcsGit = &vcsCmd{
 
 // scpSyntaxRe matches the SCP-like addresses used by Git to access
 // repositories by SSH.
-var scpSyntaxRe = regexp.MustCompile(`^(?:([a-zA-Z0-9_]+)@)?([a-zA-Z0-9._-]+):(.*)$`)
+var scpSyntaxRe = regexp.MustCompile(`^([a-zA-Z0-9_]+)@([a-zA-Z0-9._-]+):(.*)$`)
 
 func gitRemoteRepo(vcsGit *vcsCmd, rootDir string) (remoteRepo string, err error) {
 	cmd := "config remote.origin.url"
@@ -171,12 +171,10 @@ func gitRemoteRepo(vcsGit *vcsCmd, rootDir string) (remoteRepo string, err error
 		// "ssh://git@github.com/user/repo".
 		repoURL = &url.URL{
 			Scheme:  "ssh",
+			User:    url.User(m[1]),
 			Host:    m[2],
 			RawPath: m[3],
 		}
-		if m[1] != "" {
-			repoURL.User = url.User(m[1])
-		}
 	} else {
 		repoURL, err = url.Parse(out)
 		if err != nil {