Make '!' aliases more useful

Make '!' aliases more useful

When an alias starts with an exclamation mark, the rest is interpreted
as a shell command. However, all arguments passed to git used to be
ignored.

Now you can have an alias like

	$ git config alias.e '!echo'

and

	$ git e Hello World

does what you expect it to do.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>