sha1_file: remove recursion in unpack_entry

sha1_file: remove recursion in unpack_entry

Similar to the recursion in packed_object_info(), this leads to
problems on stack-space-constrained systems in the presence of long
delta chains.

We proceed in three phases:

1. Dig through the delta chain, saving each delta object's offsets and
   size on an ad-hoc stack.

2. Unpack the base object at the bottom.

3. Unpack and apply the deltas from the stack.

Signed-off-by: Thomas Rast <redacted>
Signed-off-by: Junio C Hamano <redacted>