path/filepath: ignore dot for Dir(`\\server\share`)

Dir(`\\server\share`) returns `\\server\share.`. Change Dir so it
returns `\\server\share` instead.

Fixes #18783

Change-Id: I9e0dd71ea6aea85e6c6114aaa4bb3bea3270d818
Reviewed-on: https://go-review.googlesource.com/35690Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/path/filepath/path.go

@@ -461,6 +461,10 @@ func Dir(path string) string {
 		i--
 	}
 	dir := Clean(path[len(vol) : i+1])
+	if dir == "." && len(vol) > 2 {
+		// must be UNC
+		return vol
+	}
 	return vol + dir
 }
 

src/path/filepath/path_test.go

@@ -665,6 +665,7 @@ var windirtests = []PathTest{
 	{`c:\a\b`, `c:\a`},
 	{`c:a\b`, `c:a`},
 	{`c:a\b\c`, `c:a\b`},
+	{`\\host\share`, `\\host\share`},
 	{`\\host\share\`, `\\host\share\`},
 	{`\\host\share\a`, `\\host\share\`},
 	{`\\host\share\a\b`, `\\host\share\a`},