t990X: use '.git/objects' as 'deep inside .git' path

t990X: use '.git/objects' as 'deep inside .git' path

Tests t9902-completion.sh and t9903-bash-prompt.sh each have tests
that check what happens when we are "in the '.git' directory" and
when we are "deep inside the '.git' directory".

To test the case when we are "deep inside the '.git' directory" the
test scripts used to perform a `cd .git/refs/heads`.

As there are plans to implement other ref storage systems, let's
use '.git/objects' instead of '.git/refs/heads' as the "deep inside
the '.git' directory" path.

This makes it clear to readers that these tests do not depend on
which ref backend is used.

The internals of the loose refs backend are still tested in
t1400-update-ref.sh.

Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>