strbuf_read(): skip unnecessary strbuf_grow() at eof

The loop in strbuf_read() uses xread() repeatedly while extending the strbuf until the call returns zero. If the buffer is sufficiently large to begin with, this results in xread() returning the remainder of the file to the end (returning non-zero), the loop extending the strbuf, and then making another call to xread() to have it return zero. By using read_in_full(), we can tell when the read reached the end of file: when it returns less than was requested, it's eof. This way we can avoid an extra iteration that allocates an extra 8kB that is never used. Signed-off-by: Jim Hill <gjthill@gmail.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>