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)

runtime/doc/usr_41.txt

@@ -883,6 +883,7 @@ Command line:					*command-line-functions*
 	getcmdtype()		return the current command-line type
 	getcmdwintype()		return the current command-line window type
 	getcompletion()		list of command-line completion matches
+	fullcommand()		get full command name
 
 Quickfix and location lists:			*quickfix-functions*
 	getqflist()		list of quickfix errors