completion: remove credential helpers from porcelain commands

Don't offer the "main" 'git credential' command or any of the credential
helpers from contrib/credential/ when completing git commands.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -665,8 +665,8 @@ __git_list_porcelain_commands ()
 		checkout-index)   : plumbing;;
 		commit-tree)      : plumbing;;
 		count-objects)    : infrequent;;
-		credential-cache) : credentials helper;;
-		credential-store) : credentials helper;;
+		credential)       : credentials;;
+		credential-*)     : credentials helper;;
 		cvsexportcommit)  : export;;
 		cvsimport)        : import;;
 		cvsserver)        : daemon;;