log-tree.c: cleanup a bit append_signoff()

log-tree.c: cleanup a bit append_signoff()

This patch clean up append_signoff() by moving specific code that
looks up for "^[-A-Za-z]+: [^@]+@" pattern into a function.

It also stops the primary search when the cursor oversteps
'buf + at' limit.

This patch changes slightly append_signoff() behaviour too. If we
detect any Signed-off-by pattern during the primary search, we
needn't to do a pattern research after.

Signed-off-by: Franck Bui-Huu <vagabon.xyz@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>