Allow checking for merge conflicts after running a custom command

We have a use-case to rebind 'm' to the merge action in the branches panel. There's three ways to handle this:
1) For all global keybindings, define a per-panel key that invokes it
2) Give a name to all controller actions and allow them to be invoked in custom commands
3) Allow checking for merge conflicts after running a custom command so that users can add their own 'git merge' custom command
that matches the in-built action

Option 1 is hairy, Option 2 though good for users introduces new backwards compatibility issues that I don't want to do
right now, and option 3 is trivially easy to implement so that's what I'm doing.

I've put this under an 'after' key so that we can add more things later. I'm imagining other things like being able to
move the cursor to a newly added item etc.

I considered always running this hook by default but I'd rather not: it's matching on the output text and I'd rather something
like that be explicitly opted-into to avoid cases where we erroneously believe that there are conflicts.

pkg/gui/services/custom_commands/handler_creator.go

@@ -17,27 +17,30 @@ import (
 
 // takes a custom command and returns a function that will be called when the corresponding user-defined keybinding is pressed
 type HandlerCreator struct {
-	c                  *helpers.HelperCommon
-	sessionStateLoader *SessionStateLoader
-	resolver           *Resolver
-	menuGenerator      *MenuGenerator
-	suggestionsHelper  *helpers.SuggestionsHelper
+	c                    *helpers.HelperCommon
+	sessionStateLoader   *SessionStateLoader
+	resolver             *Resolver
+	menuGenerator        *MenuGenerator
+	suggestionsHelper    *helpers.SuggestionsHelper
+	mergeAndRebaseHelper *helpers.MergeAndRebaseHelper
 }
 
 func NewHandlerCreator(
 	c *helpers.HelperCommon,
 	sessionStateLoader *SessionStateLoader,
 	suggestionsHelper *helpers.SuggestionsHelper,
+	mergeAndRebaseHelper *helpers.MergeAndRebaseHelper,
 ) *HandlerCreator {
 	resolver := NewResolver(c.Common)
 	menuGenerator := NewMenuGenerator(c.Common)
 
 	return &HandlerCreator{
-		c:                  c,
-		sessionStateLoader: sessionStateLoader,
-		resolver:           resolver,
-		menuGenerator:      menuGenerator,
-		suggestionsHelper:  suggestionsHelper,
+		c:                    c,
+		sessionStateLoader:   sessionStateLoader,
+		resolver:             resolver,
+		menuGenerator:        menuGenerator,
+		suggestionsHelper:    suggestionsHelper,
+		mergeAndRebaseHelper: mergeAndRebaseHelper,
 	}
 }
 
@@ -272,7 +275,16 @@ func (self *HandlerCreator) finalHandler(customCommand config.CustomCommand, ses
 			cmdObj.StreamOutput()
 		}
 		output, err := cmdObj.RunWithOutput()
+
+		if refreshErr := self.c.Refresh(types.RefreshOptions{Mode: types.ASYNC}); err != nil {
+			self.c.Log.Error(refreshErr)
+		}
+
 		if err != nil {
+			if customCommand.After.CheckForConflicts {
+				return self.mergeAndRebaseHelper.CheckForConflicts(err)
+			}
+
 			return self.c.Error(err)
 		}
 
@@ -280,11 +292,9 @@ func (self *HandlerCreator) finalHandler(customCommand config.CustomCommand, ses
 			if strings.TrimSpace(output) == "" {
 				output = self.c.Tr.EmptyOutput
 			}
-			if err = self.c.Alert(cmdStr, output); err != nil {
-				return self.c.Error(err)
-			}
-			return self.c.Refresh(types.RefreshOptions{})
+			return self.c.Alert(cmdStr, output)
 		}
-		return self.c.Refresh(types.RefreshOptions{})
+
+		return nil
 	})
 }