patch 8.1.2262: unpack assignment in function not recognized

Problem:    Unpack assignment in function not recognized.
Solution:   Skip over "[a, b]". (closes #5051)

src/testdir/test_let.vim

@@ -303,4 +303,12 @@ E
      END
   endif
   call assert_equal([], check)
+
+  " unpack assignment
+  let [a, b, c] =<< END
+     x
+     \y
+     z
+END
+  call assert_equal(['     x', '     \y', '     z'], [a, b, c])
 endfunc

src/userfunc.c

@@ -2482,9 +2482,14 @@ ex_function(exarg_T *eap)
 	    }
 
 	    // Check for ":let v =<< [trim] EOF"
+	    //       and ":let [a, b] =<< [trim] EOF"
 	    arg = skipwhite(skiptowhite(p));
+	    if (*arg == '[')
+		arg = vim_strchr(arg, ']');
+	    if (arg != NULL)
+	    {
 		arg = skipwhite(skiptowhite(arg));
-	    if (arg[0] == '=' && arg[1] == '<' && arg[2] =='<'
+		if ( arg[0] == '=' && arg[1] == '<' && arg[2] =='<'
 			&& ((p[0] == 'l'
 				&& p[1] == 'e'
 				&& (!ASCII_ISALNUM(p[2])
@@ -2503,6 +2508,7 @@ ex_function(exarg_T *eap)
 		    is_heredoc = TRUE;
 		}
 	    }
+	}
 
 	/* Add the line to the function. */
 	if (ga_grow(&newlines, 1 + sourcing_lnum_off) == FAIL)

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2262,
 /**/
     2261,
 /**/