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>

Makefile

@@ -768,6 +768,7 @@ LIB_OBJS += reachable.o
 LIB_OBJS += read-cache.o
 LIB_OBJS += reflog-walk.o
 LIB_OBJS += refs.o
+LIB_OBJS += refs/files-backend.o
 LIB_OBJS += ref-filter.o
 LIB_OBJS += remote.o
 LIB_OBJS += replace_object.o
@@ -2419,7 +2420,7 @@ profile-clean:
 	$(RM) $(addsuffix *.gcno,$(addprefix $(PROFILE_DIR)/, $(object_dirs)))
 
 clean: profile-clean coverage-clean
-	$(RM) *.o *.res block-sha1/*.o ppc/*.o compat/*.o compat/*/*.o
+	$(RM) *.o *.res refs/*.o block-sha1/*.o ppc/*.o compat/*.o compat/*/*.o
 	$(RM) xdiff/*.o vcs-svn/*.o ewah/*.o builtin/*.o
 	$(RM) $(LIB_FILE) $(XDIFF_LIB) $(VCSSVN_LIB)
 	$(RM) $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) git$X