Factor out the 'unlink_entry()' function from unpack-trees.c to
entry.c. It will be used in other places as well in subsequent
steps.
As it's no longer a static function, also move the documentation to
the header file to make it more discoverable.
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>