Merge user keybindings with defaults about diffview.nvim

[sindrets]

I'm glad you find the plugin useful!

It was indeed a conscious decision, and my rationale is this:

• It's easy to copy the provided default config, and then remove the bindings you don't want.
• It's not so easy to unmap a default binding if it conflicts with a binding you already use.

But I'm aware that this is an uncommon design rationale in the current landscape of lua plugins. So I might just do as you suggested: making the default behavior to merge bindings, and optionally do it the way it's currently done.