sha1dc: adjust header includes for git

sha1dc: adjust header includes for git

We can replace system includes with git-compat-util.h or
cache.h (and should make sure it is included first in all C
files).  And we can drop includes from headers entirely, as
every C file should include git-compat-util.h itself.

We will add in new include guards around the header files,
though (otherwise you get into trouble including both
sha1dc/sha1.h and cache.h).

And finally, we'll use the full "sha1dc/" path for including
related files. This isn't strictly necessary, but makes the
expected resolution more obvious.

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