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/init.lua

@@ -0,0 +1,164 @@
+---@class jj.diff
+local M = {}
+
+local utils = require("jj.utils")
+
+---@alias jj.diff.backend "native"|"diffview"|"codediff"|string
+
+---@class jj.diff.current_opts
+---@field rev? string          -- revision to diff against (default: "@-")
+---@field path? string         -- path to diff (default: current buffer path)
+---@field backend? jj.diff.backend
+---@field layout? "vertical"|"horizontal" -- only used by native backend
+
+---@class jj.diff.revision_opts
+---@field rev string           -- revision to show
+---@field path? string         -- optional single-file filter
+---@field backend? jj.diff.backend
+---@field display? "floating"|"tab"|"split" -- hint to backend
+
+---@class jj.diff.revisions_opts
+---@field left string          -- left/base revision
+---@field right string         -- right/target revision
+---@field path? string         -- optional single-file filter
+---@field backend? jj.diff.backend
+---@field display? "floating"|"tab"|"split"
+
+---@class jj.diff.BackendImpl
+---@field diff_current? fun(opts: jj.diff.current_opts)
+---@field show_revision? fun(opts: jj.diff.revision_opts)
+---@field diff_revisions? fun(opts: jj.diff.revisions_opts)
+
+---@class jj.diff.config
+---@field backend? jj.diff.backend
+---@field backends? table<string, table>
+
+---@class jj.diff.diff_opts
+---@field rev string the revision to diff against
+
+---@type jj.diff.config
+M.config = {
+	backend = "native",
+	backends = {},
+}
+
+---@type table<string, jj.diff.BackendImpl>
+local backends = {}
+
+-----------------------------------------------------------------------
+-- Backend Registry
+-----------------------------------------------------------------------
+
+--- Register or override a backend implementation
+---@param name string
+---@param impl jj.diff.BackendImpl
+function M.register_backend(name, impl)
+	backends[name] = impl
+end
+
+--- Get the configured default backend name
+---@return string
+local function get_config_backend()
+	local ok, cfg = pcall(function()
+		return require("jj").config.diff
+	end)
+	if ok and cfg and cfg.backend then
+		return cfg.backend
+	end
+	return M.config.backend or "native"
+end
+
+--- Get a backend implementation by name, falling back to native
+---@param name? string
+---@return jj.diff.BackendImpl
+local function get_backend(name)
+	name = name or get_config_backend()
+	local impl = backends[name]
+	if not impl then
+		utils.notify(
+			string.format("[Diff] backend '%s' not available, falling back to 'native'", name),
+			vim.log.levels.WARN
+		)
+		impl = backends.native
+	end
+	return impl
+end
+
+--- Setup the diff module
+---@param cfg? jj.diff.config
+function M.setup(cfg)
+	M.config = vim.tbl_deep_extend("force", M.config, cfg or {})
+	-- Laod default backends
+	pcall(require, "jj.diff.diffview")
+	pcall(require, "jj.diff.codediff")
+	pcall(require, "jj.diff.native")
+end
+
+-----------------------------------------------------------------------
+-- Unified Public API
+-----------------------------------------------------------------------
+
+--- Single dispatcher (canonical entry point)
+---@param kind "current"|"revision"|"revisions"
+---@param opts table
+function M.open(kind, opts)
+	opts = opts or {}
+	local backend = get_backend(opts.backend)
+
+	if kind == "current" then
+		if backend.diff_current then
+			return backend.diff_current(opts)
+		end
+		return backends.native.diff_current(opts)
+	elseif kind == "revision" then
+		if backend.show_revision then
+			return backend.show_revision(opts)
+		end
+		return backends.native.show_revision(opts)
+	elseif kind == "revisions" then
+		if backend.diff_revisions then
+			return backend.diff_revisions(opts)
+		end
+		return backends.native.diff_revisions(opts)
+	else
+		utils.notify("[Diff] unknown diff kind: " .. tostring(kind), vim.log.levels.ERROR)
+	end
+end
+
+--- Diff current buffer against a revision
+---@param opts? jj.diff.current_opts
+function M.diff_current(opts)
+	return M.open("current", opts or {})
+end
+
+--- Show what changed in a single revision
+---@param opts jj.diff.revision_opts
+function M.show_revision(opts)
+	return M.open("revision", opts)
+end
+
+--- Diff between two revisions
+---@param opts jj.diff.revisions_opts
+function M.diff_revisions(opts)
+	return M.open("revisions", opts)
+end
+
+---
+-----------------------------------------------------------------------
+-- BACKWARDS COMPATIBLE API
+-----------------------------------------------------------------------
+
+-- Open a vertical diff split for a specific revision of the current file
+--- @param opts? jj.diff.diff_opts Any passed arguments
+function M.open_vdiff(opts)
+	M.diff_current(vim.tbl_extend("force", { layout = "vertical" }, { rev = opts and opts.rev }))
+end
+
+-- Open a horizontal diff split for a specific revision of the current file
+--- @param opts? jj.diff.diff_opts Any passed arguments
+function M.open_hdiff(opts)
+	M.diff_current(vim.tbl_extend("force", { layout = "horizontal" }, { rev = opts and opts.rev }))
+end
+
+---@return jj.diff
+return M