http: extract http_reauth_prepare() from retry paths

http: extract http_reauth_prepare() from retry paths

All three HTTP retry paths (http_request_recoverable, post_rpc,
probe_rpc) call credential_fill() directly when handling
HTTP_REAUTH. Extract this into a helper function so that a
subsequent commit can add pre-fill logic (such as attempting
empty-auth before prompting) in one place.

No functional change.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>