builtin-rm: fix index lock file path

builtin-rm: fix index lock file path

When hold_locked_index() is called with a relative git_dir and you are
outside the work tree, the lock file become relative to the current
directory. So when later setup_work_tree() change the current directory
it breaks lock file path and commit_locked_index() fails.

This patch move index locking code after setup_work_tree() call to make
lock file relative to the working tree as it should be and add a test
case.

Noticed by Nick Andrew.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>