builtin/log.c: separate default and setup of cmd_log_init()

builtin/log.c: separate default and setup of cmd_log_init()

cmd_log_init() sets up some default rev options and then calls
setup_revisions(), so that a caller cannot set up own defaults: Either
they get overriden by cmd_log_init() (if set before) or they override
the command line (if set after). We even complain about this in a
comment to cmd_log_reflog().

Therefore, separate the two steps so that one can still call
cmd_log_init() or, alternatively, cmd_log_init_defaults() followed by
cmd_log_init_finish() (and set defaults in between).

No functional change so far.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>