Match absolute patterns without volume on windows

In 1.1.5 Glob() on windows matched absolute patterns without volume specified with the current working dir's volume.
This broke in 1.2.x due to the use of filepath.IsAbs() which returns false if the volume is not present.
This was causing paths starting with / or \ on windows to be erroneously evaluated from the current directory.

doublestar.go

@@ -336,7 +336,7 @@ func Glob(pattern string) (matches []string, err error) {
 	// volumeName will be an empty string. If it is absolute and we're on a
 	// Windows machine, volumeName will be a drive letter ("C:") for filesystem
 	// paths or \\<server>\<share> for UNC paths.
-	isAbs := filepath.IsAbs(pattern)
+	isAbs := filepath.IsAbs(pattern) || pattern[0] == '\\' || pattern[0] == '/'
 	volumeName := filepath.VolumeName(pattern)
 	isWindowsUNC := strings.HasPrefix(volumeName, `\\`)
 	if isWindowsUNC || isAbs {

doublestar_test.go

@@ -201,6 +201,10 @@ func TestGlob(t *testing.T) {
 			// test both relative paths and absolute paths
 			testGlobWith(t, idx, tt, "test")
 			testGlobWith(t, idx, tt, abspath)
+			volumeName := filepath.VolumeName(abspath)
+			if volumeName != "" && !strings.HasPrefix(volumeName, `\\`) {
+				testGlobWith(t, idx, tt, strings.TrimPrefix(abspath, volumeName))
+			}
 		}
 	}
 }