format-patch: mark rev_info with UNLEAK

format-patch: mark rev_info with UNLEAK

The comand uses a single instance of rev_info on stack, makes a
single revision traversal and exit.  Mark the resources held by the
rev_info structure with UNLEAK().

We do not do this at lower level in revision.c or cmd_log_walk(), as
a new caller of the revision traversal API can make unbounded number
of rev_info during a single run, and UNLEAK() would not a be
suitable mechanism to deal with such a caller.

Signed-off-by: Junio C Hamano <gitster@pobox.com>