rev-list --bisect: limit list before bisecting.

I noticed bisect does not work well without both good and bad.
Running this script in git.git repository would give you quite
different results:

	#!/bin/sh
        initial=e83c5163316f89bfbde7d9ab23ca2e25604af290

        mid0=`git rev-list --bisect ^$initial --all`

        git rev-list $mid0 | wc -l
        git rev-list ^$mid0 --all | wc -l

        mid1=`git rev-list --bisect --all`

        git rev-list $mid1 | wc -l
        git rev-list ^$mid1 --all | wc -l

The $initial commit is the very first commit you made.  The
first midpoint bisects things evenly as designed, but the latter
does not.

The reason I got interested in this was because I was wondering
if something like the following would help people converting a
huge repository from foreign SCM, or preparing a repository to
be fetched over plain dumb HTTP only:

        #!/bin/sh

        N=4
        P=.git/objects/pack
        bottom=

        while test 0 \< $N
        do
                N=$((N-1))
                if test -z "$bottom"
                then
                        newbottom=`git rev-list --bisect --all`
                else
                        newbottom=`git rev-list --bisect ^$bottom --all`
                fi
                if test -z "$bottom"
                then
                        rev_list="$newbottom"
                elif test 0 = $N
                then
                        rev_list="^$bottom --all"
                else
                        rev_list="^$bottom $newbottom"
                fi
                p=$(git rev-list --unpacked --objects $rev_list |
                    git pack-objects $P/pack)
                git show-index <$P/pack-$p.idx | wc -l
                bottom=$newbottom
        done

The idea is to pack older half of the history to one pack, then
older half of the remaining history to another, to continue a
few times, using finer granularity as we get closer to the tip.

This may not matter, since for a truly huge history, running
bisect number of times could be quite time consuming, and we
might be better off running "git rev-list --all" once into a
temporary file, and manually pick cut-off points from the
resulting list of commits.  After all we are talking about
"approximately half" for such an usage, and older history does
not matter much.

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

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