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.

docs/Custom_Command_Keybindings.md

@@ -59,6 +59,12 @@ For a given custom command, here are the allowed fields:
 | description | Label for the custom command when displayed in the keybindings menu | no |
 | stream | Whether you want to stream the command's output to the Command Log panel | no |
 | showOutput | Whether you want to show the command's output in a popup within Lazygit | no |
+| after | Actions to take after the command has completed | no |
+
+Here are the options for the `after` key:
+| _field_ | _description_ | required |
+|-----------------|----------------------|-|
+| checkForConflicts | true/false. If true, check for merge conflicts | no |
 
 ## Contexts