Enforce single-item selection in various actions

We want to show an error when the user tries to invoke an action that expects only
a single item to be selected.

We're using the GetDisabledReason field to enforce this (as well as DisabledReason
on menu items).

I've created a ListControllerTrait to store some shared convenience functions for this.

pkg/gui/controllers/list_controller_trait.go

@@ -0,0 +1,95 @@
+package controllers
+
+import "github.com/jesseduffield/lazygit/pkg/gui/types"
+
+// Embed this into your list controller to get some convenience methods for
+// ensuring a single item is selected, etc.
+
+type ListControllerTrait[T comparable] struct {
+	c           *ControllerCommon
+	context     types.IListContext
+	getSelected func() T
+}
+
+func NewListControllerTrait[T comparable](
+	c *ControllerCommon,
+	context types.IListContext,
+	getSelected func() T,
+) *ListControllerTrait[T] {
+	return &ListControllerTrait[T]{
+		c:           c,
+		context:     context,
+		getSelected: getSelected,
+	}
+}
+
+// Convenience function for combining multiple disabledReason callbacks.
+// The first callback to return a disabled reason will be the one returned.
+func (self *ListControllerTrait[T]) require(callbacks ...func() *types.DisabledReason) func() *types.DisabledReason {
+	return func() *types.DisabledReason {
+		for _, callback := range callbacks {
+			if disabledReason := callback(); disabledReason != nil {
+				return disabledReason
+			}
+		}
+
+		return nil
+	}
+}
+
+// Convenience function for enforcing that a single item is selected.
+// Also takes callbacks for additional disabled reasons, and passes the selected
+// item into each one.
+func (self *ListControllerTrait[T]) singleItemSelected(callbacks ...func(T) *types.DisabledReason) func() *types.DisabledReason {
+	return func() *types.DisabledReason {
+		if self.context.GetList().AreMultipleItemsSelected() {
+			return &types.DisabledReason{Text: self.c.Tr.RangeSelectNotSupported}
+		}
+
+		var zeroValue T
+		item := self.getSelected()
+		if item == zeroValue {
+			return &types.DisabledReason{Text: self.c.Tr.NoItemSelected}
+		}
+
+		for _, callback := range callbacks {
+			if reason := callback(item); reason != nil {
+				return reason
+			}
+		}
+
+		return nil
+	}
+}
+
+// Passes the selected item to the callback. Used for handler functions.
+func (self *ListControllerTrait[T]) withItem(callback func(T) error) func() error {
+	return func() error {
+		var zeroValue T
+		commit := self.getSelected()
+		if commit == zeroValue {
+			return self.c.ErrorMsg(self.c.Tr.NoItemSelected)
+		}
+
+		return callback(commit)
+	}
+}
+
+// Like withItem, but doesn't show an error message if no item is selected.
+// Use this for click actions (it's a no-op to click empty space)
+func (self *ListControllerTrait[T]) withItemGraceful(callback func(T) error) func() error {
+	return func() error {
+		var zeroValue T
+		commit := self.getSelected()
+		if commit == zeroValue {
+			return nil
+		}
+
+		return callback(commit)
+	}
+}
+
+// All controllers must implement this method so we're defining it here for convenience
+func (self *ListControllerTrait[T]) Context() types.Context {
+	return self.context
+}