commit: More generous accepting of RFC-2822 footer lines.

commit: More generous accepting of RFC-2822 footer lines.

'git commit -s' will insert a blank line before the Signed-off-by
line at the end of the message, unless this last line is a
Signed-off-by line itself.  Common use has other trailing lines
at the ends of commit text, in the style of RFC2822 headers.

Be more generous in considering lines to be part of this footer.
If the last paragraph of the commit message reasonably resembles
RFC-2822 formatted lines, don't insert that blank line.

The new Signed-off-by line is still only suppressed when the
author's existing Signed-off-by is the last line of the message.

Signed-off-by: David Brown <davidb@quicinc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>