path/filepath: handle c: as first parameter in Join properly

This is CL 11882 brought back to life.

Fixes #11551

Change-Id: I29810183957745442d1e9937f56a66fc9c6cc82a
Reviewed-on: https://go-review.googlesource.com/17470Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/path/filepath/path_test.go

@@ -267,7 +267,12 @@ var winjointests = []JoinTest{
 	{[]string{`C:\Windows\`, `System32`}, `C:\Windows\System32`},
 	{[]string{`C:\Windows\`, ``}, `C:\Windows`},
 	{[]string{`C:\`, `Windows`}, `C:\Windows`},
-	{[]string{`C:`, `Windows`}, `C:\Windows`},
+	{[]string{`C:`, `a`}, `C:a`},
+	{[]string{`C:`, `a\b`}, `C:a\b`},
+	{[]string{`C:`, `a`, `b`}, `C:a\b`},
+	{[]string{`C:.`, `a`}, `C:a`},
+	{[]string{`C:a`, `b`}, `C:a\b`},
+	{[]string{`C:a`, `b`, `d`}, `C:a\b\d`},
 	{[]string{`\\host\share`, `foo`}, `\\host\share\foo`},
 	{[]string{`\\host\share\foo`}, `\\host\share\foo`},
 	{[]string{`//host/share`, `foo/bar`}, `\\host\share\foo\bar`},

src/path/filepath/path_windows.go

@@ -120,6 +120,11 @@ func join(elem []string) string {
 
 // joinNonEmpty is like join, but it assumes that the first element is non-empty.
 func joinNonEmpty(elem []string) string {
+	if len(elem[0]) == 2 && elem[0][1] == ':' {
+		// First element is drive leter without terminating slash.
+		// Keep path relative to current directory on that drive.
+		return Clean(elem[0] + strings.Join(elem[1:], string(Separator)))
+	}
 	// The following logic prevents Join from inadvertently creating a
 	// UNC path on Windows. Unless the first element is a UNC path, Join
 	// shouldn't create a UNC path. See golang.org/issue/9167.