refs: introduce `refs_for_each_include_root_refs()`

refs: introduce `refs_for_each_include_root_refs()`

Introduce a new ref iteration flag `DO_FOR_EACH_INCLUDE_ROOT_REFS`,
which will be used to iterate over regular refs plus pseudorefs and
HEAD.

Refs which fall outside the `refs/` and aren't either pseudorefs or HEAD
are more of a grey area. This is because we don't block the users from
creating such refs but they are not officially supported.

Introduce `refs_for_each_include_root_refs()` which calls
`do_for_each_ref()` with this newly introduced flag.

In `refs/files-backend.c`, introduce a new function
`add_pseudoref_and_head_entries()` to add pseudorefs and HEAD to the
`ref_dir`. We then finally call `add_pseudoref_and_head_entries()`
whenever the `DO_FOR_EACH_INCLUDE_ROOT_REFS` flag is set. Any new ref
backend will also have to implement similar changes on its end.

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