[PATCH 0/5, resend] fork/exec removal series

Here is a series of patches that removes a number fork/exec pairs. They are replaced by delegating to start_command/finish_command/run_command.

I've addressed all issues that were raised. However, concerning whether git_connect() shoud return NULL or not, I decided to stay with my earlier approach to return NULL for non-forking connections. Consequently, I had to remove all error checks because there is now no unique value that can indicate failure. The motivation is that the alternate approach (to always return non-NULL for any successful connection and NULL for failure) would benefit *only* libification; but *if* that ever happens, a major audit (and possibly) rewrite in the callers has to be done anyway because currently we are juggling, leaking, and double-closing file descriptors like mad - in error paths and normal paths.

Patch 2 depends on Patch 1; otherwise, there are no dependencies. The series goes on top of 'master'. Note that 'next' has an additional call of git_connect() in the new transport.c.

 builtin-archive.c |    8 +--
 cache.h           |    4 +-
 connect.c         |  128 +++++++++++++++++++++++++---------------------------
 convert.c         |   30 +++----------
 diff.c            |   38 +--------------
 fetch-pack.c      |   43 +++++------------
 peek-remote.c     |    8 +--
 send-pack.c       |    8 +--
 8 files changed, 96 insertions(+), 171 deletions(-)

-- Hannes