[PATCH 0/3] introduce GIT_CACHE_DIRECTORY to plumbing

[Zach Welch <zw@superlucidity.net>]

The first patch introduces the GIT_CACHE_DIRECTORY to the C plumbing.
Without this patch, the index file and its lock are always placed
in './.git'.  Scripts wishing to run these commands from a different 
working directory can use this support to override the cache directory.

The second patch renames the DB_ENVIRONMENT symbols to match.  Note,
the name changes came from an off-list comment from Linus, after my
sendpatches script accidentally bombed him with a half dozen copies of
the init-db patch series summary.  His comment inspired these last two.

The third patch renames SHA1_FILE_DIRECTORY as GIT_OBJECT_DIRECTORY,
to match the "GIT_CACHE_DIRECTORY" name introduced in the first patch.
This was done last and seperately so it can be applied after a little
bit of notice.  We're not at a point of need to be backwards compatible.

This require my latest init-db cleanups be applied first, otherwise
you will get a rejection in init-db.c.

There are 3 patches in this series:
        [PATCH 1/3] add GIT_CACHE_DIRECTORY support
        [PATCH 2/3] rename object directory symbols
        [PATCH 3/3] rename SHA1_FILE_DIRECTORY