Enable smart HTTP transport to place EOF at end of pack

Enable smart HTTP transport to place EOF at end of pack

When fetching over smart HTTP the InputStream that gets fed into
a PackParser doesn't really support EOF at the end of the pack. It
instead tries to make a new HTTP request, which fails because there
is no request body currently buffered by the client.

Make EOF work correctly on the end of an HTTP derived InputStream
for the pack by denoting no more requests are expected as the higher
level code is now consuming the pack (or side-band embedded pack).
Smart HTTP support doesn't automatically enqueue execute support onto
the end of the UnionInputStream, which allows the UnionInputStream
to correctly reflect EOF when the HTTP response is consumed.

Change-Id: I975f1ab1c81ab1c1af925716970088bc7b8d6b1a