patch 9.0.0285: it is not easy to change the command line from a plugin

Problem:    It is not easy to change the command line from a plugin.
Solution:   Add setcmdline(). (Shougo Matsushita, closes #10869)

runtime/doc/builtin.txt

@@ -505,6 +505,7 @@ setbufvar({buf}, {varname}, {val})
 setcellwidths({list})		none	set character cell width overrides
 setcharpos({expr}, {list})	Number	set the {expr} position to {list}
 setcharsearch({dict})		Dict	set character search from {dict}
+setcmdline({str} [, {pos}])	Number	set command-line
 setcmdpos({pos})		Number	set cursor position in command-line
 setcursorcharpos({list})	Number	move cursor to position in {list}
 setenv({name}, {val})		none	set environment variable
@@ -3425,7 +3426,8 @@ getcmdcompltype()					*getcmdcompltype()*
 		Only works when the command line is being edited, thus
 		requires use of |c_CTRL-\_e| or |c_CTRL-R_=|.
 		See |:command-completion| for the return string.
-		Also see |getcmdtype()|, |setcmdpos()| and |getcmdline()|.
+		Also see |getcmdtype()|, |setcmdpos()|, |getcmdline()| and
+		|setcmdline()|.
 		Returns an empty string when completion is not defined.
 
 getcmdline()						*getcmdline()*
@@ -3434,7 +3436,8 @@ getcmdline()						*getcmdline()*
 		|c_CTRL-R_=|.
 		Example: >
 			:cmap <F7> <C-\>eescape(getcmdline(), ' \')<CR>
-<		Also see |getcmdtype()|, |getcmdpos()| and |setcmdpos()|.
+<		Also see |getcmdtype()|, |getcmdpos()|, |setcmdpos()| and
+		|setcmdline()|.
 		Returns an empty string when entering a password or using
 		|inputsecret()|.
 
@@ -3444,7 +3447,8 @@ getcmdpos()						*getcmdpos()*
 		Only works when editing the command line, thus requires use of
 		|c_CTRL-\_e| or |c_CTRL-R_=| or an expression mapping.
 		Returns 0 otherwise.
-		Also see |getcmdtype()|, |setcmdpos()| and |getcmdline()|.
+		Also see |getcmdtype()|, |setcmdpos()|, |getcmdline()| and
+		|setcmdline()|.
 
 getcmdscreenpos()					*getcmdscreenpos()*
 		Return the screen position of the cursor in the command line
@@ -3453,7 +3457,8 @@ getcmdscreenpos()					*getcmdscreenpos()*
 		Only works when editing the command line, thus requires use of
 		|c_CTRL-\_e| or |c_CTRL-R_=| or an expression mapping.
 		Returns 0 otherwise.
-		Also see |getcmdpos()|, |setcmdpos()|.
+		Also see |getcmdpos()|, |setcmdpos()|, |getcmdline()| and
+		|setcmdline()|.
 
 getcmdtype()						*getcmdtype()*
 		Return the current command-line type. Possible return values
@@ -7925,6 +7930,16 @@ setcharsearch({dict})					*setcharsearch()*
 		Can also be used as a |method|: >
 			SavedSearch()->setcharsearch()
 
+setcmdline({str} [, {pos}])					*setcmdline()*
+		Set the command line to {str} and set the cursor position to
+		{pos}.
+		If {pos} is omitted, the cursor is positioned after the text.
+		Returns 0 when successful, 1 when not editing the command
+		line.
+
+		Can also be used as a |method|: >
+			GetText()->setcmdline()
+
 setcmdpos({pos})					*setcmdpos()*
 		Set the cursor position in the command line to byte position
 		{pos}.  The first position is 1.
@@ -7937,8 +7952,8 @@ setcmdpos({pos})					*setcmdpos()*
 		before inserting the resulting text.
 		When the number is too big the cursor is put at the end of the
 		line.  A number smaller than one has undefined results.
-		Returns FALSE when successful, TRUE when not editing the
-		command line.
+		Returns 0 when successful, 1 when not editing the command
+		line.
 
 		Can also be used as a |method|: >
 			GetPos()->setcmdpos()

runtime/doc/usr_41.txt

@@ -1038,6 +1038,7 @@ Command line:					*command-line-functions*
 	getcmdpos()		get position of the cursor in the command line
 	getcmdscreenpos()	get screen position of the cursor in the
 				command line
+	setcmdline()		set the current command line
 	setcmdpos()		set position of the cursor in the command line
 	getcmdtype()		return the current command-line type
 	getcmdwintype()		return the current command-line window type