refactor(lua): rename vim.diff => vim.text.diff #34864

Problem:
`vim.diff()` was introduced before we had the `vim.text` module, where
it obviously belongs.

Solution:
Move it.

src/gen/gen_vimdoc.lua

@@ -133,7 +133,6 @@ local config = {
     filename = 'lua.txt',
     section_order = {
       'hl.lua',
-      'diff.lua',
       'mpack.lua',
       'json.lua',
       'base64.lua',
@@ -185,7 +184,6 @@ local config = {
       'runtime/lua/vim/text.lua',
       'runtime/lua/vim/glob.lua',
       'runtime/lua/vim/_meta/builtin.lua',
-      'runtime/lua/vim/_meta/diff.lua',
       'runtime/lua/vim/_meta/mpack.lua',
       'runtime/lua/vim/_meta/json.lua',
       'runtime/lua/vim/_meta/base64.lua',
@@ -230,7 +228,6 @@ local config = {
           'mpack',
           'json',
           'base64',
-          'diff',
           'spell',
           'regex',
           'lpeg',

src/nvim/lua/stdlib.c

@@ -761,9 +761,9 @@ void nlua_state_add_stdlib(lua_State *const lstate, bool is_thread)
   lua_setfield(lstate, -2, "lpeg");
   lua_pop(lstate, 4);
 
-  // vim.diff
+  // vim.text.diff
   lua_pushcfunction(lstate, &nlua_xdl_diff);
-  lua_setfield(lstate, -2, "diff");
+  lua_setfield(lstate, -2, "_diff");
 
   // vim.json
   lua_cjson_new(lstate);

src/nvim/lua/xdiff.c

@@ -164,9 +164,7 @@ static int call_on_hunk_cb(int start_a, int count_a, int start_b, int count_b, v
   lua_pushinteger(lstate, count_b);
 
   if (lua_pcall(lstate, 4, 1, 0) != 0) {
-    api_set_error(err, kErrorTypeException,
-                  "error running function on_hunk: %s",
-                  lua_tostring(lstate, -1));
+    api_set_error(err, kErrorTypeException, "on_hunk: %s", lua_tostring(lstate, -1));
     return -1;
   }