[PATCH 0/4] Introduce the .git-file

This is a resend of my previous patches + an update to make git-submodule
work with the .git-file + a litte documentation.

PS: Regarding the color of the bikeshed, it seems that everyone can live
with  "gitdir:" so that's what I ended up using.

Diffstat:
 Documentation/repository-layout.txt |    5 ++-
 git-submodule.sh                    |    4 +-
 setup.c                             |   40 +++++++++++++++++++
 t/t0002-gitfile.sh                  |   71 +++++++++++++++++++++++++++++++++++
 4 files changed, 117 insertions(+), 3 deletions(-)