contrib/credential: avoid fixed-size buffer in osxkeychain

The macOS Keychain-based credential helper reads the newline-delimited
protocol stream one line at a time by repeatedly calling fgets() into a
fixed-size buffer, and is thus affected by the vulnerability described
in the previous commit.

To mitigate this attack, avoid using a fixed-size buffer, and instead
rely on getline() to allocate a buffer as large as necessary to fit the
entire content of the line, preventing any protocol injection.

We solved a similar problem in a5bb10fd5e (config: avoid fixed-sized
buffer when renaming/deleting a section, 2023-04-06) by switching to
strbuf_getline(). We can't do that here because the contrib helpers do
not link with the rest of Git, and so can't use a strbuf. But we can use
the system getline() directly, which works similarly.

In most parts of Git we don't assume that every platform has getline().
But this helper is run only on OS X, and that platform added support in
10.7 ("Lion") which was released in 2011.

Tested-by: Taylor Blau <me@ttaylorr.com>
Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
