cmd/go: read URL not Repository Root from svn info

This makes custom import path checks work even when the
custom import metadata directs checking out a subtree
of the subversion repository.

(Git and Mercurial allow no such thing, so they are unaffected.)

Fixes #20731.

Change-Id: I635f3a2037d69a87c6dac7b08b0a0d8266abd250
Reviewed-on: https://go-review.googlesource.com/46417
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -302,15 +302,20 @@ func svnRemoteRepo(vcsSvn *vcsCmd, rootDir string) (remoteRepo string, err error
 	out := string(outb)
 
 	// Expect:
+	//
 	//	 ...
-	// Repository Root: <URL>
+	// 	URL: <URL>
 	// 	...
-
-	i := strings.Index(out, "\nRepository Root: ")
+	//
+	// Note that we're not using the Repository Root line,
+	// because svn allows checking out subtrees.
+	// The URL will be the URL of the subtree (what we used with 'svn co')
+	// while the Repository Root may be a much higher parent.
+	i := strings.Index(out, "\nURL: ")
 	if i < 0 {
 		return "", fmt.Errorf("unable to parse output of svn info")
 	}
-	out = out[i+len("\nRepository Root: "):]
+	out = out[i+len("\nURL: "):]
 	i = strings.Index(out, "\n")
 	if i < 0 {
 		return "", fmt.Errorf("unable to parse output of svn info")