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/native.lua

@@ -0,0 +1,127 @@
+local utils = require("jj.utils")
+local buffer = require("jj.core.buffer")
+
+local diff = require("jj.diff")
+
+--- Get the content of a file at a specific revision
+--- @param rev string The revision
+--- @param path string The file path
+--- @return table lines The file content
+local function get_file_content(rev, path)
+	local cmd = string.format("jj file show -r %s %s", vim.fn.shellescape(rev), vim.fn.shellescape(path))
+	local content = vim.fn.system(cmd)
+	local success = vim.v.shell_error == 0
+	if success then
+		return vim.split(content, "\n", { trimempty = true })
+	else
+		return {}
+	end
+end
+
+--- Open a read-only buffer for a specific revision of a file
+--- @param rev string The revision
+--- @param path string The file path
+local function open_revision(rev, path)
+	local lines = get_file_content(rev, path)
+
+	local buf = vim.api.nvim_create_buf(false, true)
+
+	local buf_name = string.format("jj://%s/%s", rev, path)
+	vim.api.nvim_buf_set_name(buf, buf_name)
+	vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
+
+	local ft = vim.filetype.match({ filename = path })
+	if ft then
+		vim.bo[buf].filetype = ft
+	end
+
+	vim.bo[buf].buftype = "nofile"
+	vim.bo[buf].bufhidden = "wipe"
+	vim.bo[buf].readonly = true
+	vim.bo[buf].swapfile = false
+	vim.bo[buf].modifiable = true
+
+	vim.api.nvim_win_set_buf(0, buf)
+end
+
+-----------------------------------------------------------------------
+-- Native Backend
+-----------------------------------------------------------------------
+
+diff.register_backend("native", {
+	--- Opens a side-by-side diff of the current buffer against a revision.
+	--- Creates a split with the revision content on the left and the current buffer on the right.
+	--- Closing either side will clean up both and restore the original cursor position.
+	diff_current = function(opts)
+		if not utils.ensure_jj() then
+			return
+		end
+
+		-- Save current state to restore after diff is closed
+		local prev_buf = vim.api.nvim_get_current_buf()
+		local prev_cur_pos = buffer.get_cursor(prev_buf) or { 1, 0 }
+
+		local rev = opts.rev or "@-"
+		local path = opts.path or vim.api.nvim_buf_get_name(0)
+		local layout = opts.layout or "vertical"
+
+		local split_fun = layout == "horizontal" and vim.cmd.split or vim.cmd.vsplit
+		local orig_win = vim.api.nvim_get_current_win()
+
+		-- Use better diff algorithm for code moves and indentation
+		local saved_diffopt = vim.o.diffopt
+		vim.opt.diffopt:append("algorithm:patience,indent-heuristic")
+
+		-- Set up diff: current buffer on right, revision on left
+		vim.cmd.diffthis()
+		split_fun({ mods = { split = "aboveleft" } })
+		open_revision(rev, path)
+		vim.cmd.diffthis()
+
+		local rev_buf = vim.api.nvim_get_current_buf()
+		local augroup = vim.api.nvim_create_augroup("JJDiffCleanup" .. rev_buf, { clear = true })
+
+		-- Cleanup closes both sides, exits diff mode, and restores cursor.
+		local function cleanup()
+			vim.api.nvim_del_augroup_by_id(augroup)
+			vim.schedule(function()
+				if vim.api.nvim_buf_is_valid(rev_buf) then
+					vim.api.nvim_buf_delete(rev_buf, { force = true })
+				end
+				if vim.api.nvim_win_is_valid(orig_win) then
+					vim.api.nvim_set_current_win(orig_win)
+					vim.cmd.diffoff()
+				end
+				buffer.set_cursor(prev_buf, prev_cur_pos)
+				vim.o.diffopt = saved_diffopt
+			end)
+		end
+
+		-- Trigger cleanup when either the revision buffer or original window is closed
+		vim.api.nvim_create_autocmd({ "BufWipeout", "BufHidden" }, {
+			group = augroup,
+			buffer = rev_buf,
+			once = true,
+			callback = cleanup,
+		})
+
+		vim.api.nvim_create_autocmd("WinClosed", {
+			group = augroup,
+			pattern = tostring(orig_win),
+			once = true,
+			callback = cleanup,
+		})
+	end,
+	show_revision = function(opts)
+		local terminal = require("jj.ui.terminal")
+
+		local cmd = string.format("jj show -r %s --quiet --no-pager", opts.rev)
+		terminal.run_floating(cmd, require("jj.cmd").floating_keymaps())
+	end,
+	diff_revisions = function(opts)
+		local terminal = require("jj.ui.terminal")
+
+		local cmd = string.format("jj diff -f %s -t %s --quiet --no-pager", opts.left, opts.right)
+		terminal.run_floating(cmd, require("jj.cmd").floating_keymaps())
+	end,
+})