cmd/go: report correct directory for 'no version control'

The scan starts at the directory we care about and works
backward to the GOPATH root. The error should say the
original directory name, not the name of the GOPATH root.

Fixes #6175.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/13366050

src/cmd/go/test.bash

@@ -139,6 +139,18 @@ if ! ./testgo test ./testdata/testimport/*.go; then
 	ok=false
 fi
 
+TEST version control error message includes correct directory
+export GOPATH=$(pwd)/testdata/shadow/root1
+if ./testgo get -u foo 2>testdata/err; then
+	echo "go get -u foo succeeded unexpectedly"
+	ok=false
+elif ! grep testdata/shadow/root1/src/foo testdata/err >/dev/null; then
+	echo "go get -u error does not mention shadow/root1/src/foo:"
+	cat testdata/err
+	ok=false
+fi
+unset GOPATH
+
 # Test that without $GOBIN set, binaries get installed
 # into the GOPATH bin directory.
 TEST install into GOPATH

src/cmd/go/vcs.go

@@ -321,6 +321,7 @@ func vcsForDir(p *Package) (vcs *vcsCmd, root string, err error) {
 		return nil, "", fmt.Errorf("directory %q is outside source root %q", dir, srcRoot)
 	}
 
+	origDir := dir
 	for len(dir) > len(srcRoot) {
 		for _, vcs := range vcsList {
 			if fi, err := os.Stat(filepath.Join(dir, "."+vcs.cmd)); err == nil && fi.IsDir() {
@@ -337,7 +338,7 @@ func vcsForDir(p *Package) (vcs *vcsCmd, root string, err error) {
 		dir = ndir
 	}
 
-	return nil, "", fmt.Errorf("directory %q is not using a known version control system", dir)
+	return nil, "", fmt.Errorf("directory %q is not using a known version control system", origDir)
 }
 
 // repoRoot represents a version control system, a repo, and a root of