patch 9.1.0099: Not able to use diff() with 'diffexpr'

Problem:  Not able to use diff() with 'diffexpr'
          (rickhowe, after v9.1.0096)
Solution: Use a default context length of 0, update diff() help text,
          add a test for using diff() with 'diffexpr'
          (Yegappan Lakshmanan)

closes: #14013

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 67a1427..d13eb1c 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1952,7 +1952,7 @@
     {"did_filetype",	0, 0, 0,	    NULL,
 			ret_number_bool,    f_did_filetype},
     {"diff",		2, 3, FEARG_1,	    arg3_diff,
-			ret_list_dict_any,  f_diff},
+			ret_any,  f_diff},
     {"diff_filler",	1, 1, FEARG_1,	    arg1_lnum,
 			ret_number,	    f_diff_filler},
     {"diff_hlID",	2, 2, FEARG_1,	    arg2_lnum_number,