patch 8.2.3226: new digraph functions use old naming scheme

Problem:    New digraph functions use old naming scheme.
Solution:   Use the digraph_ prefix. (Hirohito Higashi, closes #8580)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 1091e56..2a9c972 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1292,6 +1292,14 @@
 			ret_number,	    f_diff_filler},
     {"diff_hlID",	2, 2, FEARG_1,	    arg2_lnum_number,
 			ret_number,	    f_diff_hlID},
+    {"digraph_get",	1, 1, FEARG_1,	    arg1_string,
+			ret_string,	    f_digraph_get},
+    {"digraph_getlist",0, 1, FEARG_1,	    arg1_number,
+			ret_list_string_items, f_digraph_getlist},
+    {"digraph_set",	2, 2, FEARG_1,	    arg2_string_number,
+			ret_bool,	f_digraph_set},
+    {"digraph_setlist",1, 1, FEARG_1,	    arg1_list_string,
+			ret_bool,	    f_digraph_setlist},
     {"echoraw",		1, 1, FEARG_1,	    arg1_string,
 			ret_void,	    f_echoraw},
     {"empty",		1, 1, FEARG_1,	    NULL,
@@ -1406,10 +1414,6 @@
 			ret_list_number,    f_getcursorcharpos},
     {"getcwd",		0, 2, FEARG_1,	    arg2_number,
 			ret_string,	    f_getcwd},
-    {"getdigraph",     1, 1, FEARG_1,      arg1_string,
-                        ret_string,         f_getdigraph},
-    {"getdigraphlist",  0, 1, FEARG_1,      arg1_number,
-                        ret_list_string_items, f_getdigraphlist},
     {"getenv",		1, 1, FEARG_1,	    arg1_string,
 			ret_any,	    f_getenv},
     {"getfontname",	0, 1, 0,	    arg1_string,
@@ -1870,10 +1874,6 @@
 			ret_number_bool,    f_setcmdpos},
     {"setcursorcharpos", 1, 3, FEARG_1,	    arg13_cursor,
 			ret_number_bool,    f_setcursorcharpos},
-    {"setdigraph",	2, 2, FEARG_1,	    arg2_string_number,
-			ret_bool,           f_setdigraph},
-    {"setdigraphlist",	1, 1, FEARG_1,	    arg1_list_string,
-			ret_bool,	    f_setdigraphlist},
     {"setenv",		2, 2, FEARG_2,	    arg2_string_any,
 			ret_void,	    f_setenv},
     {"setfperm",	2, 2, FEARG_1,	    arg2_string,