git-credential-osxkeychain has largely fallen behind other external
credential helpers in the features it supports, and hasn't received any
functional changes since 2013. As it stood, osxkeychain failed seven tests
in the external credential helper test suite:

not ok 8 - helper (osxkeychain) overwrites on store
not ok 9 - helper (osxkeychain) can forget host
not ok 11 - helper (osxkeychain) does not erase a password distinct from input
not ok 15 - helper (osxkeychain) erases all matching credentials
not ok 18 - helper (osxkeychain) gets password_expiry_utc
not ok 19 - helper (osxkeychain) overwrites when password_expiry_utc changes
not ok 21 - helper (osxkeychain) gets oauth_refresh_token

After this set of patches, osxkeychain passes all tests in the external

credential helper test suite.

Great work!

Could these tests run as part of macOS CI?