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/eval.txt

@@ -2562,6 +2562,7 @@ foldlevel({lnum})		Number	fold level at {lnum}
 foldtext()			String	line displayed for closed fold
 foldtextresult({lnum})		String	text for closed fold at {lnum}
 foreground()			Number	bring the Vim window to the foreground
+fullcommand({name})		String	get full command from {name}
 funcref({name} [, {arglist}] [, {dict}])
 				Funcref	reference to function {name}
 function({name} [, {arglist}] [, {dict}])
@@ -4902,6 +4903,21 @@ foreground()	Move the Vim window to the foreground.  Useful when sent from
 		{only in the Win32, Athena, Motif and GTK GUI versions and the
 		Win32 console version}
 
+fullcommand({name})						*fullcommand()*
+		Get the full command name from a short abbreviated command
+		name; see |20.2| for details on command abbreviations.
+
+		{name} may start with a `:` and can include a [range], these
+		are skipped and not returned.
+		Returns an empty string if a command doesn't exist or if it's
+		ambiguous (for user-defined functions).
+
+		For example `fullcommand('s')`, `fullcommand('sub')`,
+		`fullcommand(':%substitute')` all return "substitute".
+
+		Can also be used as a |method|: >
+			GetName()->fullcommand()
+<
 						*funcref()*
 funcref({name} [, {arglist}] [, {dict}])
 		Just like |function()|, but the returned Funcref will lookup

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