[PATCH 0/5 v2] Introduce the .gitfile

The patch-series has been reordered and updated according to comments on the earlier series.

Shortlog:
 Simplify setup of $GIT_DIR in git-sh-setup.sh
 Add platform-independent .git "symlink"
 Teach resolve_gitlink_ref() about the .git file
 git-submodule: prepare for the .git-file
 Teach GIT-VERSION-GEN about the .git file

Diffstat:
 Documentation/repository-layout.txt |    5 ++-
 GIT-VERSION-GEN                     |    2 +-
 cache.h                             |    1 +
 environment.c                       |    2 +
 git-sh-setup.sh                     |    8 +---
 git-submodule.sh                    |    4 +-
 refs.c                              |   17 +++++++-
 setup.c                             |   47 ++++++++++++++++++++++
 t/t0002-gitfile.sh                  |   74 +++++++++++++++++++++++++++++++++++
 9 files changed, 146 insertions(+), 14 deletions(-)