connect.c: mark die_initial_contact() NORETURN

connect.c: mark die_initial_contact() NORETURN

There is a series running in parallel with this one that adds code
like this

    switch (...) {
    case ...:
        die_initial_contact();
    case ...:

There is nothing wrong with this. There is no actual falling
through. But since gcc is not that smart and gcc 7.x introduces
-Wimplicit-fallthrough, it raises a false alarm in this case.

This class of warnings may be useful elsewhere, so instead of
suppressing the whole class, let's try to fix just this code. gcc is
smart enough to realize that no execution can continue after a
NORETURN function call and no longer raises the warning.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>