refs/files: refactor `add_pseudoref_and_head_entries()`

refs/files: refactor `add_pseudoref_and_head_entries()`

The `add_pseudoref_and_head_entries()` function accepts both the ref
store as well as a directory name as input. This is unnecessary though
as the ref store already uniquely identifies the root directory of the
ref store anyway.

Furthermore, the function is misnamed now that we have clarified the
meaning of pseudorefs as it doesn't add pseudorefs, but root refs.
Rename it accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>