These flags were split between cache.h and packfile.h,
because some of the flags apply only to packs. However, they
share a single numeric namespace, since both are respected
for the packed variant. Let's make sure they're defined
together so that nobody accidentally adds a new flag in one
location that duplicates the other.
While we're here, let's also put them in an enum (which
helps debugger visibility) and use "(1<<n)" rather than
counting powers of 2 manually.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>