Simplify setup of $GIT_DIR in git-sh-setup.sh
Simplify setup of $GIT_DIR in git-sh-setup.sh

Using 'git rev-parse --git-dir' makes the code shorter and more future-
proof.

Signed-off-by: Lars Hjemli <1022a68b169538dabe33b1e3a08dffa0cefd6c22@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git