git-p4: support git worktrees

git-p4: support git worktrees

git-p4 would attempt to find the git directory using
its own specific code, which did not know about git
worktrees.

Rework it to use "git rev-parse --git-dir" instead.

Add test cases for worktree usage and specifying
git directory via --git-dir and $GIT_DIR.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>