credential: erase all matching credentials

credential: erase all matching credentials

`credential reject` sends the erase action to each helper, but the
exact behaviour of erase isn't specified in documentation or tests.
Some helpers (such as credential-store and credential-libsecret) delete
all matching credentials, others (such as credential-cache) delete at
most one matching credential.

Test that helpers erase all matching credentials. This behaviour is
easiest to reason about. Users expect that `echo
"url=https://example.com" | git credential reject` or `echo
"url=https://example.com\nusername=tim" | git credential reject` erase
all matching credentials.

Fix credential-cache.

Signed-off-by: M Hickford <mirth.hickford@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>