refs/refs-internal.h: new header file

refs/refs-internal.h: new header file

There are a number of constants, structs, and static functions defined
in refs.c and treated as private to the references module. But we want
to support multiple reference backends within the reference module,
and those backends will need access to some heretofore private
declarations.

We don't want those declarations to be visible to non-refs code, so we
don't want to move them to refs.h. Instead, add a new header file,
refs/refs-internal.h, that is intended to be included only from within
the refs module. Make some functions non-static and move some
declarations (and their corresponding docstrings) from refs.c to this
file.

In a moment we will add more content to the "refs" subdirectory.

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