credential: new attribute password_expiry_utc

Some passwords have an expiry date known at generation. This may be
years away for a personal access token or hours for an OAuth access
token.

When multiple credential helpers are configured, `credential fill` tries
each helper in turn until it has a username and password, returning
early. If Git authentication succeeds, `credential approve`
stores the successful credential in all helpers. If authentication
fails, `credential reject` erases matching credentials in all helpers.
Helpers implement corresponding operations: get, store, erase.

The credential protocol has no expiry attribute, so helpers cannot
store expiry information. Even if a helper returned an improvised
expiry attribute, git credential discards unrecognised attributes
between operations and between helpers.

This is a particular issue when a storage helper and a
credential-generating helper are configured together:

	[credential]
		helper = storage  # eg. cache or osxkeychain
		helper = generate  # eg. oauth

`credential approve` stores the generated credential in both helpers
without expiry information. Later `credential fill` may return an
expired credential from storage. There is no workaround, no matter how
clever the second helper. The user sees authentication fail (a retry
will succeed).

Introduce a password expiry attribute. In `credential fill`, ignore
expired passwords and continue to query subsequent helpers.

In the example above, `credential fill` ignores the expired password
and a fresh credential is generated. If authentication succeeds,
`credential approve` replaces the expired password in storage.
If authentication fails, the expired credential is erased by
`credential reject`. It is unnecessary but harmless for storage
helpers to self prune expired credentials.

Add support for the new attribute to credential-cache.
Eventually, I hope to see support in other popular storage helpers.

Example usage in a credential-generating helper
https://github.com/hickford/git-credential-oauth/pull/16

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

builtin/credential-cache--daemon.c

@@ -127,6 +127,9 @@ static void serve_one_client(FILE *in, FILE *out)
 		if (e) {
 			fprintf(out, "username=%s\n", e->item.username);
 			fprintf(out, "password=%s\n", e->item.password);
+			if (e->item.password_expiry_utc != TIME_MAX)
+				fprintf(out, "password_expiry_utc=%"PRItime"\n",
+					e->item.password_expiry_utc);
 		}
 	}
 	else if (!strcmp(action.buf, "exit")) {