fix usage string for git grep

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>