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

No test because the code has no test.

Fixes #12313.

Change-Id: I2cfd0a0422c0cd76f0371c2d3bbbdf5bb3b3f1eb
Reviewed-on: https://go-review.googlesource.com/17951Reviewed-by: Brad Fitzpatrick <bradfitz@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"
@@ -171,10 +171,12 @@ 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 {