Teach revision machinery about --no-walk

Teach revision machinery about --no-walk

The flag "no_walk" is present in struct rev_info since a long time, but
so far has been in use exclusively by "git show".

With this flag, you can see all your refs, ordered by date of the last
commit:

$ git log --abbrev-commit --pretty=oneline --decorate --all --no-walk

which is extremely helpful if you have to juggle with a lot topic
branches, and do not remember in which one you introduced that uber
debug option, or simply want to get an overview what is cooking.

(Note that the "git log" invocation above does not output the same as

 $ git show --abbrev-commit --pretty=oneline --decorate --all --quiet

 since "git show" keeps the alphabetic order that "--all" returns the
 refs in, even if the option "--date-order" was passed.)

For good measure, this also adds the "--do-walk" option which overrides
"--no-walk".

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>