Don't discard the drive letter when joining with an absolute path

When encountering a symlink that contains an absolute target without a
drive letter, assume the target resides on the same drive.

pkg/filesystem/path/builder.go

@@ -189,9 +189,10 @@ func (w *buildingScopeWalker) OnAbsolute() (ComponentWalker, error) {
 		return nil, err
 	}
 	*w.b = Builder{
-		absolute:   true,
-		components: w.b.components[:0],
-		suffix:     "/",
+		absolute:    true,
+		driveLetter: w.b.driveLetter,
+		components:  w.b.components[:0],
+		suffix:      "/",
 	}
 	return w.b.getComponentWalker(componentWalker), nil
 }

pkg/filesystem/path/builder_test.go

@@ -325,4 +325,28 @@ func TestBuilder(t *testing.T) {
 		require.NoError(t, path.Resolve(path.NewUNIXParser(".."), s2))
 		require.Equal(t, "/hello/..", builder2.GetUNIXString())
 	})
+
+	// When encountering a symlink target that is an absolute path
+	// without a drive letter, we should assume the path resolves to
+	// a location on the same drive.
+	t.Run("DriveLetterWithAbsoluteSymlink", func(t *testing.T) {
+		scopeWalker1 := mock.NewMockScopeWalker(ctrl)
+		componentWalker1 := mock.NewMockComponentWalker(ctrl)
+		scopeWalker1.EXPECT().OnDriveLetter('C').Return(componentWalker1, nil)
+		scopeWalker2 := mock.NewMockScopeWalker(ctrl)
+		componentWalker1.EXPECT().OnTerminal(path.MustNewComponent("hello")).Return(
+			&path.GotSymlink{
+				Parent: scopeWalker2,
+				Target: path.NewWindowsParser("\\world"),
+			},
+			nil,
+		)
+		componentWalker2 := mock.NewMockComponentWalker(ctrl)
+		scopeWalker2.EXPECT().OnAbsolute().Return(componentWalker2, nil)
+		componentWalker2.EXPECT().OnTerminal(path.MustNewComponent("world"))
+
+		builder1, s1 := path.EmptyBuilder.Join(scopeWalker1)
+		require.NoError(t, path.Resolve(path.NewWindowsParser("C:\\hello"), s1))
+		require.Equal(t, "C:\\world", mustGetWindowsString(builder1))
+	})
 }