Avoid a useless prefix lookup in strbuf_expand()

Avoid a useless prefix lookup in strbuf_expand()

Currently, the --pretty=format prefix is looked up in a
tight loop in strbuf_expand(), if prefix is found it is then
used as argument for format_commit_item() that does another
search by a switch statement to select the proper operation.

Because the switch statement is already able to discard
unknown matches we don't need the prefix lookup before
to call format_commit_item().

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>