Intoducing the .git file (again)

These patches enables .git to be a textfile containing the path to the git
directory proper. It passes all the tests so hopefully there are no
regressions, but there may be bugs and omissions lurking when this feature
is actually used; I've exercised it in my git, cgit and dayjob repos but git
is big and has many codepaths so I wouldn't be suprised if there still are
some git commands left which fails to obey the .git file.

PS: These patches could certainly be squashed into a single patch, but I've
left them as is to make each one easier to review.

PPS: If included, the .git file should probably be used by git-submodule to
clone submodule repositories into something like $GIT_DIR/submodules/<name>,
as that would make local submodule changes more resistant to dataloss due to
checkout/reset in the containing repository.

Shortlog:
 Add platform-independent .git "symlink"
 Fix setup of $GIT_DIR in git-sh-setup.sh
 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                       |   38 ++++++++++++++++++
 git-sh-setup.sh                     |   12 ++---
 git-submodule.sh                    |    4 +-
 refs.c                              |   17 +++++++-
 setup.c                             |    9 ++++
 t/t0002-gitfile.sh                  |   74 +++++++++++++++++++++++++++++++++++
 9 files changed, 148 insertions(+), 14 deletions(-)