completion: zsh: improve main function selection

Sometimes we want to use the function directly (e.g. _git_checkout), for
example when zsh has the option 'complete_aliases', this way, we can do
something like:

  compdef _git gco=git_checkout

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

git-completion.zsh

@@ -226,8 +226,10 @@ _git ()
 
 	if (( $+functions[__${service}_zsh_main] )); then
 		__${service}_zsh_main
-	else
+	elif (( $+functions[__${service}_main] )); then
 		emulate ksh -c __${service}_main
+	elif (( $+functions[_${service}] )); then
+		emulate ksh -c _${service}
 	fi
 
 	let _ret && _default && _ret=0