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>

t/t9800-git-p4-basic.sh

@@ -257,6 +257,26 @@ test_expect_success 'submit from detached head' '
 	)
 '
 
+test_expect_success 'submit from worktree' '
+	test_when_finished cleanup_git &&
+	git p4 clone --dest="$git" //depot &&
+	(
+		cd "$git" &&
+		git worktree add ../worktree-test
+	) &&
+	(
+		cd "$git/../worktree-test" &&
+		test_commit "worktree-commit" &&
+		git config git-p4.skipSubmitEdit true &&
+		git p4 submit
+	) &&
+	(
+		cd "$cli" &&
+		p4 sync &&
+		test_path_is_file worktree-commit.t
+	)
+'
+
 test_expect_success 'kill p4d' '
 	kill_p4d
 '