refs.c: move dwim_ref()/dwim_log() from sha1_name.c

Both dwim_ref()/dwim_log() functions are intimately related to the ref
parsing rules defined in refs.c and better fits there. Move them together
with substitute_branch_name(), a file scope static helper function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>