streaming: free git_istream upon closing

Kirill Smelkov noticed that post-1.7.6 "git checkout"
started leaking tons of memory. The streaming_write_entry
function properly calls close_istream(), but that function
did not actually free() the allocated git_istream struct.

The git_istream struct is totally opaque to calling code,
and must be heap-allocated by open_istream. Therefore it's
not appropriate for callers to have to free it.

This patch makes close_istream() into "close and de-allocate
all associated resources". We could add a new "free_istream"
call, but there's not much point in letting callers inspect
the istream after close. And this patch's semantics make us
match fopen/fclose, which is well-known and understood.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>