use uppercase POSIX compliant signals for the 'trap' command

use uppercase POSIX compliant signals for the 'trap' command

In 'man 1p trap' there is written:

    "Implementations may permit names with the SIG prefix or ignore case
    in signal names as an extension."

So change the lowercase signals to uppercase, which is POSIX compliant
instead of being an extension.

There wasn't anybody claiming that it doesn't work, but there was a bug
with using a signal with the SIG prefix, which is an extension as well.
So let's play it safe and change it, since it doesn't hurt anyone.

While at it, also convert 8 indentation spaces to 1 tab character.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>