Speed up git-svn fetch

I first sent this patch series 6 months ago today [1], then resent it after some comments from Junio, Johannes, and Brian [2]. Here it is again, addressing further comments from Junio and Eric.

The point of the series is to speed up git-svn fetch on Cygwin, where forking is quite slow. In my informal testing, this patch seems to speed things up by ~1.4-~1.7x. We accomplish this by having a single long-lived git-cat-file process and a single long-lived git-hash-object process for the duration of the git-svn invocation.

This series is based on top of next.

-Adam

--
 Documentation/git-cat-file.txt    |   43 +++++++-
 Documentation/git-hash-object.txt |    5 +-
 builtin-cat-file.c                |  153 ++++++++++++++++++++++++---
 git-svn.perl                      |   42 ++++----
 hash-object.c                     |   44 ++++++++-
 perl/Git.pm                       |  208 ++++++++++++++++++++++++++++++++++++-
 t/t1006-cat-file.sh               |  181 ++++++++++++++++++++++++++++++++
 t/t1007-hash-object.sh            |  139 +++++++++++++++++++++++++
 t/t5303-hash-object.sh            |   35 ------
 9 files changed, 768 insertions(+), 82 deletions(-)