cmd_merge(): Parse options before checking MERGE_HEAD

cmd_merge(): Parse options before checking MERGE_HEAD

Reorder the initial part of builtin/merge.c:cmd_merge() so that command-line
options are parsed _before_ we load the index and check for MERGE_HEAD
(and exits if it exists). This does not change the behaviour of 'git merge',
but is needed in preparation for the implementation of 'git merge --abort'
(which requires MERGE_HEAD to be present).

This patch has been improved by the following contributions:
- Junio C Hamano: fixup minor style issues

Thanks-to: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>