completion: remove 'git column' from porcelain commands

'git column' is an internal helper, so it should not be offered on
'git <TAB>' along with porcelain commands.
Signed-off-by: NSZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -663,6 +663,7 @@ __git_list_porcelain_commands ()
 		check-mailmap)    : plumbing;;
 		check-ref-format) : plumbing;;
 		checkout-index)   : plumbing;;
+		column)           : internal helper;;
 		commit-tree)      : plumbing;;
 		count-objects)    : infrequent;;
 		credential-cache) : credentials helper;;