revert: do not pass non-literal string as format to git_path()

This fixes the following warning.

CC builtin/revert.o
builtin/revert.c: In function ‘write_cherry_pick_head’:
builtin/revert.c:311: warning: format not a string literal and no format arguments

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>