Introduce GIT_DIR environment variable.

Introduce GIT_DIR environment variable.

During the mailing list discussion on renaming GIT_ environment
variables, people felt that having one environment that lets the
user (or Porcelain) specify both SHA1_FILE_DIRECTORY (now
GIT_OBJECT_DIRECTORY) and GIT_INDEX_FILE for the default layout
would be handy.  This change introduces GIT_DIR environment
variable, from which the defaults for GIT_INDEX_FILE and
GIT_OBJECT_DIRECTORY are derived.  When GIT_DIR is not defined,
it defaults to ".git".  GIT_INDEX_FILE defaults to
"$GIT_DIR/index" and GIT_OBJECT_DIRECTORY defaults to
"$GIT_DIR/objects".

Special thanks for ideas and discussions go to Petr Baudis and
Daniel Barkalow.  Bugs are mine ;-)

Signed-off-by: Junio C Hamano <junkio@cox.net>