server_supports_v2(): use a separate function for die_on_error

server_supports_v2(): use a separate function for die_on_error

The server_supports_v2() helper lets a caller find out if the server
supports a feature, and will optionally die if it's not supported. This
makes the return value confusing, as it's only meaningful when the
function is not asked to die.

Coverity flagged a new call like:

  /* check that we support "foo" */
  server_supports_v2("foo", 1);

complaining that we usually checked the return value, but this time we
didn't. But this call is correct, and other ones that did:

  if (server_supports_v2("foo", 1))
          do_something_with_foo();

are "wrong", in the sense that we know the conditional will always be
true (but there's no bug; the code is simply misleading).

Let's split the "die" behavior into its own function which returns void,
and modify each caller to use the correct one.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>