path/filepath: change IsAbs to treat \\host\share as an absolute path

Fixes #47123

Change-Id: I2226b8a9ea24cd88171acfbaffea2566309416de
Reviewed-on: https://go-review.googlesource.com/c/go/+/334809
Trust: Alex Brainman <alex.brainman@gmail.com>
Trust: Hajime Hoshi <hajimehoshi@gmail.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/path/filepath/path_test.go

@@ -791,6 +791,8 @@ var winisabstests = []IsAbsTest{
 	{`c:a\b`, false},
 	{`c:\a\b`, true},
 	{`c:/a/b`, true},
+	{`\\host\share`, true},
+	{`\\host\share\`, true},
 	{`\\host\share\foo`, true},
 	{`//host/share/foo/bar`, true},
 }

src/path/filepath/path_windows.go

@@ -45,6 +45,10 @@ func IsAbs(path string) (b bool) {
 	if l == 0 {
 		return false
 	}
+	// If the volume name starts with a double slash, this is a UNC path.
+	if isSlash(path[0]) && isSlash(path[1]) {
+		return true
+	}
 	path = path[l:]
 	if path == "" {
 		return false