stop using HEAD for attributes in bare repository by default

stop using HEAD for attributes in bare repository by default

With 23865355 (attr: read attributes from HEAD when bare repo,
2023-10-13), we started to use the HEAD tree as the default
attribute source in a bare repository.  One argument for such a
behaviour is that it would make things like "git archive" run in
bare and non-bare repositories for the same commit consistent.
This changes was merged to Git 2.43 but without an explicit mention
in its release notes.

It turns out that this change destroys performance of shallowly
cloning from a bare repository.  As the "server" installations are
expected to be mostly bare, and "git pack-objects", which is the
core of driving the other side of "git clone" and "git fetch" wants
to see if a path is set not to delta with blobs from other paths via
the attribute system, the change forces the server side to traverse
the tree of the HEAD commit needlessly to find if each and every
paths the objects it sends out has the attribute that controls the
deltification.  Given that (1) most projects do not configure such
an attribute, and (2) it is dubious for the server side to honor
such an end-user supplied attribute anyway, this was a poor choice
of the default.

To mitigate the current situation, let's revert the change that uses
the tree of HEAD in a bare repository by default as the attribute
source.  This will help most people who have been happy with the
behaviour of Git 2.42 and before.

Two things to note:

 * If you are stuck with versions of Git 2.43 or newer, that is
   older than the release this fix appears in, you can explicitly
   set the attr.tree configuration variable to point at an empty
   tree object, i.e.

	$ git config attr.tree 4b825dc642cb6eb9a060e54bf8d69288fbee4904

 * If you like the behaviour we are reverting, you can explicitly
   set the attr.tree configuration variable to HEAD, i.e.

	$ git config attr.tree HEAD

The right fix for this is to optimize the code paths that allow
accesses to attributes in tree objects, but that is a much more
involved change and is left as a longer-term project, outside the
scope of this "first step" fix.

Signed-off-by: Junio C Hamano <gitster@pobox.com>