Teach mailinfo %< as an alternative scissors mark

Teach mailinfo %< as an alternative scissors mark

Handle perforations found “in the wild” more robustly by recognizing
“%<” as an alternative scissors mark.

This feature is only meant to support old habits.  Discourage new use
of the percent-based version by only documenting the 8< symbol so new
users’ perforations can still be recognized by old versions of Git.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>