fix usage string for git grep

Without this patch, git-grep gives confusing usage information: $ git grep --confused usage: git grep <option>* <rev>* [-e] <pattern> [<path>...] $ git grep HEAD pattern fatal: ambiguous argument 'pattern': unknown revision or path no t in the working tree. Use '--' to separate paths from revisions So put <pattern> before the <rev>s, in accordance with actual correct usage. While we're changing the usage string, we might as well include the "--" separating revisions and paths, too. Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>