history: initialize rev_info in cmd_history_reword()

history: initialize rev_info in cmd_history_reword()

git history reword expects a single valid revision argument and errors
out if it doesn't get it.  In that case the struct rev_info passed to
release_revisions() for cleanup is still uninitialized, which can result
in attempts to free(3) random pointers.  Avoid that by initializing the
structure.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>