xutils: Fix hashing an incomplete line with whitespaces at the end

Upon seeing a whitespace, xdl_hash_record_with_whitespace() first skipped the run of whitespaces (excluding LF) that begins there, ensuring that the pointer points at the last whitespace character in the run, and assumed that the next character must be LF at the end of the line. This does not work when hashing an incomplete line, which lacks the LF at the end. Introduce "at_eol" variable that is true when either we are at the end of line (looking at LF) or at the end of an incomplete line, and use that instead throughout the code. Noticed by Thell Fowler. Signed-off-by: Junio C Hamano <gitster@pobox.com>