41467: Correct completion of 'tmux new <TAB>'.

Reference: window_pane_spawn() in tmux 2.5.

Also, document _cmdstring and _precommand.

ChangeLog

@@ -1,5 +1,9 @@
 2017-07-28  Daniel Shahaf  <d.s@daniel.shahaf.name>
 
+	* 41467: Completion/Unix/Command/_tmux,
+	Completion/Unix/Type/_cmdambivalent, Doc/Zsh/compsys.yo:
+	Correct completion of 'tmux new <TAB>'.
+
 	* 41471 (tweaked): Completion/Unix/Command/_subversion: _svn:
 	Complete propvals in 'propset'.
 

Completion/Unix/Command/_tmux

@@ -526,7 +526,7 @@ _tmux-new-session() {
     '-t+[specify target session]:session:__tmux-sessions' \
     '-x[specify width]:width' \
     '-y[specify height]:height' \
-    '*:: :_cmdstring'
+    '*:: :_cmdambivalent'
 }
 
 _tmux-new-window() {

Completion/Unix/Type/_cmdambivalent

@@ -0,0 +1,17 @@
+#autoload
+
+if (( CURRENT == 1 && ${#words} == 1 )); then
+  # Heuristics to decide whether to complete for system() or for execl().
+  local space=' '
+  if (( ${${words[CURRENT]}[(I)$space]} )); then
+    _cmdstring
+  elif [[ ${${compstate[all_quotes]}[1]} == (\'|\") ]]; then
+    _cmdstring
+  else
+    _command_names -e
+  fi
+elif (( CURRENT == 1 )); then
+  _command_names -e
+else
+  _normal
+fi

Doc/Zsh/compsys.yo

@@ -3014,6 +3014,22 @@ tt(-n), tt(-F), tt(-X) are passed to tt(compadd).
 
 See tt(_description) for a description of var(tag) and var(descr).
 )
+findex(_cmdambivalent)
+item(tt(_cmdambivalent))(
+Completes the remaining positional arguments as an external command.
+The external command and its arguments are completed as separate arguments
+(in a manner appropriate for completing tt(/usr/bin/env))
+if there are two or more remaining positional arguments on the command line,
+and as a quoted command string (in the manner of tt(system+LPAR()...+RPAR())) otherwise.
+See also tt(_cmdstring) and tt(_precommand).
+
+This function takes no arguments.
+)
+findex(_cmdstring)
+item(tt(_cmdstring))(
+Completes an external command as a single argument, as for
+tt(system+LPAR()...+RPAR()).
+)
 findex(_complete)
 item(tt(_complete))(
 This completer generates all possible completions in a context-sensitive
@@ -3214,6 +3230,11 @@ tt(old-menu), see
 ifzman(the section `Completion System Configuration' above)\
 ifnzman(noderef(Completion System Configuration)).
 )
+findex(_precommand)
+item(tt(_precommand))(
+Complete an external command in word-separated arguments, as for
+tt(exec) and tt(/usr/bin/env).
+)
 findex(_prefix)
 item(tt(_prefix))(
 This completer can be used to try completion with the suffix (everything