This makes the core code aware of delta objects and undeltafy them as
needed.  The convention is to use read_sha1_file() to have
undeltafication done automatically (most users do that already so this
is transparent).

If the delta object itself has to be accessed then it must be done
through map_sha1_file() and unpack_sha1_file().

In that context mktag.c has been switched to read_sha1_file() as there
is no reason to do the full map+unpack manually.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>