git-am: cope better with an empty Subject: line

git-am: cope better with an empty Subject: line

When the Subject: line is empty for whatever reason, git-am was fooled by
it and left an empty line at the beginning of the resulting commit log
message.

This moves the logic around so that we do not keep $SUBJECT in a separate
variable.  Instead, $dotest/msg-clean, which used to be the log message
body extracted from the message and then trailing whitespaces cleansed
out, now contains the subject line followed by a blank line at the
beginning for normal messages, and we use the first line from the file as
the summary line throughout the program.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>