xgethostname: handle long hostnames

xgethostname: handle long hostnames

If the full hostname doesn't fit in the buffer supplied to
gethostname, POSIX does not specify whether the buffer will be
null-terminated, so to be safe, we should do it ourselves.  Introduce
new function, xgethostname, which ensures that there is always a \0
at the end of the buffer.

Signed-off-by: David Turner <dturner@twosigma.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>