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/diff/codediff.lua

@@ -0,0 +1,64 @@
+local utils = require("jj.utils")
+
+---@type jj.diff
+local diff = require("jj.diff")
+
+--- Givewn two changes, show their diff using codediff
+--- @param left string
+--- @param right string
+local function diff_two_changes(left, right)
+	local commit_id_left = utils.get_commit_id(left)
+	if commit_id_left == nil then
+		return
+	end
+
+	local commit_id_right = utils.get_commit_id(right)
+	if commit_id_right == nil then
+		return
+	end
+
+	vim.cmd(string.format("CodeDiff %s %s", commit_id_right, commit_id_left))
+end
+
+-----------------------------------------------------------------------
+-- Codediff Backend
+-----------------------------------------------------------------------
+
+diff.register_backend("codediff", {
+	diff_current = function(opts)
+		if not utils.has_dependency("codediff") then
+			return
+		end
+
+		-- Extract the commit id from opts.rev
+		local commit_id = "HEAD~1"
+		if opts.rev then
+			local t_commit_id = utils.get_commit_id(opts.rev)
+			if t_commit_id == nil then
+				return
+			end
+			commit_id = t_commit_id
+		end
+
+		vim.cmd(string.format("CodeDiff file %s", commit_id))
+	end,
+
+	show_revision = function(opts)
+		if not utils.has_dependency("codediff") then
+			return
+		end
+
+		-- When comparing a revision we always compare it to it's parent to get the diff
+		local right = string.format("%s-", opts.rev)
+
+		diff_two_changes(opts.rev, right)
+	end,
+
+	diff_revisions = function(opts)
+		if not utils.has_dependency("codediff") then
+			return
+		end
+
+		diff_two_changes(opts.left, opts.right)
+	end,
+})