patch 8.2.2468: not easy to get the full command name from a shortened one

Problem:    Not easy to get the full command name from a shortened one.
Solution:   Add fullcommand(). (Martin Tournoij, closes #7777)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f15139f..01994b0 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -978,6 +978,8 @@
 			ret_string,	    f_foldtextresult},
     {"foreground",	0, 0, 0,	    NULL,
 			ret_void,	    f_foreground},
+    {"fullcommand",	1, 1, FEARG_1,	    arg1_string,
+			ret_string,	    f_fullcommand},
     {"funcref",		1, 3, FEARG_1,	    NULL,
 			ret_func_any,	    f_funcref},
     {"function",	1, 3, FEARG_1,	    NULL,