Call dmon_unwatch when changing project

Fix a conspicuous omission to call the dmon_unwatch function
when changing project directory.

This uncovered a bug or a quirk of the dmon library where the watch_ids
can change as a result of calling dmon_unwatch because they are just
indexes on a contiguous array. Use a workaround to always unwatch the
first valid watch_id N times.

src/api/system.c

@@ -723,6 +723,13 @@ static int f_watch_dir(lua_State *L) {
   return 1;
 }
 
+static int f_unwatch_dir(lua_State *L) {
+  dmon_watch_id watch_id;
+  watch_id.id = luaL_checkinteger(L, 1);
+  dmon_unwatch(watch_id);
+  return 0;
+}
+
 #if __linux__
 static int f_watch_dir_add(lua_State *L) {
   dmon_watch_id watch_id;
@@ -826,6 +833,7 @@ static const luaL_Reg lib[] = {
   { "fuzzy_match",         f_fuzzy_match         },
   { "set_window_opacity",  f_set_window_opacity  },
   { "watch_dir",           f_watch_dir           },
+  { "unwatch_dir",         f_unwatch_dir         },
   { "path_compare",        f_path_compare        },
 #if __linux__
   { "watch_dir_add",       f_watch_dir_add       },