Make default pre-commit hook less noisy

Make default pre-commit hook less noisy

This hook thought to have found a conflict marker any time it saw
a 7-character combination of any of the characters '<>=' at the
beginning of a line, whereas it should only look for the *same*
character to appear repeatedly.

Also, restrict it to match exactly 7 times, to avoid matching the
underlining with '='-characters often used in documentation.

Signed-off-by: Jean-Luc Herren <jlh@gmx.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>