fix: add cli binding for show

lua/gitsigns/actions.lua

@@ -1240,6 +1240,10 @@ M.show = function(revision, callback)
   diffthis.show(bufnr, revision, callback)
 end
 
+C.show = function(args, _)
+  M.show(args[1])
+end
+
 CP.show = complete_heads
 
 --- @param buf_or_filename string|integer

lua/gitsigns/async.lua

@@ -181,6 +181,7 @@ function M.create(argc_or_func, func)
 
   return function(...)
     local callback = argc and select(argc + 1, ...) or nil
+    assert(not callback or type(callback) == 'function')
     return run(func, callback, unpack({ ... }, 1, argc))
   end
 end