rev-list: add optional progress reporting

rev-list: add optional progress reporting

It's easy to ask rev-list to do a traversal that may takes
many seconds (e.g., by calling "--objects --all"). In theory
you can monitor its progress by the output you get to
stdout, but this isn't always easy.

Some operations, like "--count", don't make any output until
the end.

And some callers, like check_everything_connected(), are
using it just for the error-checking of the traversal, and
throw away stdout entirely.

This patch adds a "--progress" option which can be used to
give some eye-candy for a user waiting for a long traversal.
This is just a rev-list option and not a regular traversal
option, because it needs cooperation from the callbacks in
builtin/rev-list.c to do the actual count.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>