Fix gfg function completion definition

Related: #5179

plugins/git/git.plugin.zsh

@@ -88,7 +88,7 @@ alias gdw='git diff --word-diff'
 alias gf='git fetch'
 alias gfa='git fetch --all --prune'
 function gfg() { git ls-files | grep $@ }
-compdef gfg=grep
+compdef _grep gfg
 alias gfo='git fetch origin'
 
 alias gg='git gui citool'