patch, 4 partsModularize http, commit-walker, http-fetch

This series cleans up a whole lot of flow control and makes it possible to extend commit-walkers and http (each individually) in more useful directions.

Due to the limitations on what I can test, it could use a bunch more testing before it goes in. In particular, http-push is completely untested, and I haven't really beaten on USE_CURL_MULTI, although it compiles each way.

The first patch just makes the http library code easier to work with.

The second patch turns the caller-provided global function for making delayed requests into a callback in a chain.

The third patch (posted previously) removes obsolete commit-walkers.

The fourth patch (posted previously) reworks the commit-walker framework, and makes http-fetch into a commit-walker implementation which can be used from library code. It also turns http-fetch into a (trivial) builtin which calls the walker code with the library walker.

 Makefile                      |   37 ++----
 builtin-http-fetch.c          |   77 +++++++++++++
 builtin.h                     |    1 +
 fetch.h                       |   54 ---------
 git.c                         |    3 +
 http-push.c                   |   25 +---
 http-fetch.c => http-walker.c |  245 +++++++++++++++++----------------------
 http.c                        |   50 ++++++++
 http.h                        |    7 +-
 local-fetch.c                 |  254 -----------------------------------------
 rsh.c                         |   83 -------------
 rsh.h                         |    7 -
 ssh-fetch.c                   |  166 ---------------------------
 ssh-pull.c                    |    4 -
 ssh-push.c                    |    4 -
 ssh-upload.c                  |  143 -----------------------
 fetch.c => walker.c           |   83 +++++++-------
 walker.h                      |   37 ++++++
 18 files changed, 339 insertions(+), 941 deletions(-)

	-Daniel
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I just realized I forgot to use -C, so some of these patches are probably less than trivial to review as changes. If anybody wants to be able to actually read them all without applying them, I have -C versions; if nobody does, I'd just as soon not spam the list further.

	-Daniel
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