pack-objects and rev-list status updates

[Junio C Hamano <junkio@cox.net>]

Tonight's "master" has an update to teach git-rev-list to read
list of rev arguments from its standard input.  We owe Andy
Whitcroft a big credit for starting this topic.

This required splitting out part of setup_revision() and make it
callable from the side.  Thanks to this split, it got a lot easier
to teach pack-objects to do a similar trick.  I did a few:

 - Earlier patch I sent to give command line parameters similar
   to rev-list to pack-objects was reworked on.  I chose to feed
   rev arguments from its standard input; so

	echo master..next | pack-objects --revs --stdout

   would do what you would expect.

 - A patch before that one to add --unpacked=<existing pack>
   option was resurrected and further reworked on.  Now you can
   say,

	rev-list --objects --unpacked=$active --all |
        pack-objects $new

   and/or

	pack-objects --unpacked=$active --all $new </dev/null

   The </dev/null at the end is ugly but the command always
   reads from its standard input.  Unlike the "something like
   this?" patch, this version can take more than one such
   "pretend things in these packs are unpacked" arguments.

 - I also told pack-objects to understand --thin, so you can do
   a thin pack this:

	echo master..next | pack-objects --thin --stdout

   The equivalent expressed in the old way is:

	rev-list --objects-edge master..next |
        pack-objects --stdout

   Note that --thin is only usable with --stdout, as before.

Andy has a patch to use "rev-list --stdin" to lift the exec
arguments limit from send-pack; I tweaked it slightly and tested
it minimally.  I think the logical next step would be to use
"pack-objects --stdin" there and lose another pipe and a process.

We also should be able to do the same for upload-pack.  Now the
groundwork is more or less done, I think this is a good exercise
for a wannabe git hacker.  Hint hint...

I do not expect to be working on git tomorrow (Wednesday my
time), so I will not be merging any of the above (except the
"rev-list --stdin" change) in "next" tonight (eh, it is already
Wednesday wee morning), but they will appear in "pu".