refs: extract out `loose_fill_ref_dir_regular_file()`

Extract out the code for adding a single file to the loose ref dir as
`loose_fill_ref_dir_regular_file()` from `loose_fill_ref_dir()` in
`refs/files-backend.c`.

This allows us to use this function independently in the following
commits where we add code to also add pseudorefs to the ref dir.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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