Document the hairy gfi_unpack_entry part of fast-import
Junio pointed out this part of fast-import wasn't very clear on
initial read, and it took some time for someone who was new to
fast-import's "dirty little tricks" to understand how this was
even working. So a little bit of commentary in the proper place
may help future readers.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>