refs: split filesystem-based refs code into a new file

refs: split filesystem-based refs code into a new file

As another step in the move to pluggable reference backends, move the
code that is specific to the filesystem-based reference backend (i.e.,
the current system of storing references as loose and packed files) into
a separate file, refs/files-backend.c.

Aside from a tiny bit of file header boilerplate, this commit only moves
a subset of the code verbatim from refs.c to the new file, as can easily
be verified using patience diff:

    git diff --patience $commit^:refs.c $commit:refs.c
    git diff --patience $commit^:refs.c $commit:refs/files-backend.c

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>