pretty: support multiline subjects with format:

pretty: support multiline subjects with format:

git log --pretty=format:%s (and tformat:) used to display the first
line of the subject, unlike the other --pretty options, which would
construct a subject line from all lines of the first paragraph of
the commit message.

For consistency and increased code reuse, change format: to do the
same as the other options.

Before:
	$ git log --pretty=oneline v1.6.1 | md5sum
	7c0896d2a94fc3315a0372b9b3373a8f  -
	$ git log --pretty=tformat:"%H %s" v1.6.1 | md5sum
	298903b1c065002e15daa5329213c51f  -

After:
	$ git log --pretty=tformat:"%H %s" v1.6.1 | md5sum
	7c0896d2a94fc3315a0372b9b3373a8f  -
	$ git log --pretty=oneline v1.6.1 | md5sum
	7c0896d2a94fc3315a0372b9b3373a8f  -

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>