Keep local changes when checkout branch in worktree.

options.go

@@ -242,6 +242,11 @@ type CheckoutOptions struct {
 	// Force, if true when switching branches, proceed even if the index or the
 	// working tree differs from HEAD. This is used to throw away local changes
 	Force bool
+	// Keep, if true when switching branches, local changes (the index or the
+	// working tree changes) will be kept so that they can be committed to the
+	// target branch. Force and Keep are mutually exclusive, should not be both
+	// set to true.
+	Keep bool
 }
 
 // Validate validates the fields and sets the default values.

worktree.go

@@ -160,6 +160,8 @@ func (w *Worktree) Checkout(opts *CheckoutOptions) error {
 	ro := &ResetOptions{Commit: c, Mode: MergeReset}
 	if opts.Force {
 		ro.Mode = HardReset
+	} else if opts.Keep {
+		ro.Mode = SoftReset
 	}
 
 	if !opts.Hash.IsZero() && !opts.Create {