add git-check-ignore sub-command

This works in a similar manner to git-check-attr.

Thanks to Jeff King and Junio C Hamano for the idea:
http://thread.gmane.org/gmane.comp.version-control.git/108671/focus=108815

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -842,6 +842,7 @@ __git_list_porcelain_commands ()
 		archimport)       : import;;
 		cat-file)         : plumbing;;
 		check-attr)       : plumbing;;
+		check-ignore)     : plumbing;;
 		check-ref-format) : plumbing;;
 		checkout-index)   : plumbing;;
 		commit-tree)      : plumbing;;