sha1_file: add read_loose_object() function

It's surprisingly hard to ask the sha1_file code to open a
_specific_ incarnation of a loose object. Most of the
functions take a sha1, and loop over the various object
types (packed versus loose) and locations (local versus
alternates) at a low level.

However, some tools like fsck need to look at a specific
file. This patch gives them a function they can use to open
the loose object at a given path.

The implementation unfortunately ends up repeating bits of
related functions, but there's not a good way around it
without some major refactoring of the whole sha1_file stack.
We need to mmap the specific file, then partially read the
zlib stream to know whether we're streaming or not, and then
finally either stream it or copy the data to a buffer.

We can do that by assembling some of the more arcane
internal sha1_file functions, but we end up having to
essentially reimplement unpack_sha1_file(), along with the
streaming bits of check_sha1_signature().

Still, most of the ugliness is contained in the new
function, and the interface is clean enough that it may be
reusable (though it seems unlikely anything but git-fsck
would care about opening a specific file).

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

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