tree.c API: move read_tree() into builtin/ls-files.c

tree.c API: move read_tree() into builtin/ls-files.c

Since the read_tree() API was added around the same time as
read_tree_recursive() in 94537c78a82 (Move "read_tree()" to
"tree.c"[...], 2005-04-22) and b12ec373b8e ([PATCH] Teach read-tree
about commit objects, 2005-04-20) things have gradually migrated over
to the read_tree_recursive() version.

Now builtin/ls-files.c is the last user of this code, let's move all
the relevant code there. This allows for subsequent simplification of
it, and an eventual move to read_tree_recursive().

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>