git-log (internal): more options.

git-log (internal): more options.

This ports the following options from rev-list based git-log
implementation:

 * -<n>, -n<n>, and -n <n>.  I am still wondering if we want
    this natively supported by setup_revisions(), which already
    takes --max-count.  We may want to move them in the next
    round.  Also I am not sure if we can get away with not
    setting revs->limited when we set max-count.  The latest
    rev-list.c and revision.c in this series do not, so I left
    them as they are.

 * --pretty and --pretty=<fmt>.

 * --abbrev=<n> and --no-abbrev.

The previous commit already handles time-based limiters
(--since, --until and friends).  The remaining things that
rev-list based git-log happens to do are not useful in a pure
log-viewing purposes, and not ported:

 * --bisect (obviously).

 * --header.  I am actually in favor of doing the NUL
   terminated record format, but rev-list based one always
   passed --pretty, which defeated this option.  Maybe next
   round.

 * --parents.  I do not think of a reason a log viewer wants
   this.  The flag is primarily for feeding squashed history
   via pipe to downstream tools.

Signed-off-by: Junio C Hamano <junkio@cox.net>