revision walker: Fix --boundary when limited

revision walker: Fix --boundary when limited

This cleans up the boundary processing in the commit walker.  It

 - rips out the boundary logic from the commit walker. Placing
   "negative" commits in the revs->commits list was Ok if all we
   cared about "boundary" was the UNINTERESTING limiting case,
   but conceptually it was wrong.

 - makes get_revision_1() function to walk the commits and return
   the results as if there is no funny postprocessing flags such
   as --reverse, --skip nor --max-count.

 - makes get_revision() function the postprocessing phase:

   If reverse is given, wait for get_revision_1() to give
   everything that it would normally give, and then reverse it
   before consuming.

   If skip is given, skip that many before going further.

   If max is given, stop when we gave out that many.

   Now that we are about to return one positive commit, mark
   the parents of that commit to be potential boundaries
   before returning, iff we are doing the boundary processing.

   Return the commit.

 - After get_revision() finishes giving out all the positive
   commits, if we are doing the boundary processing, we look at
   the parents that we marked as potential boundaries earlier,
   see if they are really boundaries, and give them out.

It loses more code than it adds, even when the new gc_boundary()
function, which is purely for early optimization, is counted.

Note that this patch is purely for eyeballing and discussion
only.  It breaks git-bundle's verify logic because the logic
does not use BOUNDARY_SHOW flag for its internal computation
anymore.  After we correct it not to attempt to affect the
boundary processing by setting the BOUNDARY_SHOW flag, we can
remove BOUNDARY_SHOW from revision.h and use that bit assignment
for the new CHILD_SHOWN flag.

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