Feat: Rewrite diff module to allow for external dependencies as jj.nvim's diff tool (#74)

- Refactor diff into modular architecture (native, diffview, codediff)
- Add the possibility for users to define their own diff backend
- Support external diff tools like diffview.nvim codediff.nvim
- Enable visual mode selection to diff two changes in log buffer
- Add summary tooltip integration with terminal
- Update documentation with new diff workflows

lua/jj/cmd/init.lua

@@ -378,19 +378,18 @@ function M.diff(opts)
 		return
 	end
 
-	local cmd = "jj diff"
+	local diff_module = require("jj.diff")
 
 	if opts and opts.current then
 		local file = vim.fn.expand("%:p")
 		if file and file ~= "" then
-			cmd = string.format("%s %s", cmd, vim.fn.fnameescape(file))
+			diff_module.diff_current({ path = file })
 		else
 			utils.notify("Current buffer is not a file", vim.log.levels.ERROR)
-			return
 		end
+	else
+		diff_module.show_revision({ rev = "@" })
 	end
-
-	terminal.run(cmd, M.terminal_keymaps())
 end
 
 -- Jujutsu rebase