osxkeychain: erase matching passwords only

osxkeychain: erase matching passwords only

Other credential helpers support deleting credentials that match a
specified password. See 7144dee3ec (credential/libsecret: erase matching
creds only, 2023-07-26) and cb626f8e5c (credential/wincred: erase
matching creds only, 2023-07-26).

Support this in osxkeychain too by extracting, decrypting and comparing
the stored password before deleting.

Fixes the following test failure with osxkeychain:

    11 - helper (osxkeychain) does not erase a password distinct from
    input

Signed-off-by: Bo Anderson <mail@boanderson.me>
Signed-off-by: Junio C Hamano <gitster@pobox.com>