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>
diff --git a/git-tag-script b/git-tag-script
index ccc75dc..281d192 100755
--- a/git-tag-script
+++ b/git-tag-script
@@ -1,5 +1,9 @@
 #!/bin/sh
-object=${2:-$(cat .git/HEAD)}
+# Copyright (c) 2005 Linus Torvalds
+
+: ${GIT_DIR=.git}
+
+object=${2:-$(cat "$GIT_DIR"/HEAD)}
 type=$(git-cat-file -t $object) || exit 1
 ( echo -e "object $object\ntype $type\ntag $1\n"; cat ) > .tmp-tag
 rm -f .tmp-tag.asc