Fix memory corruption when .gitignore does not end by \n

Fix memory corruption when .gitignore does not end by \n

Commit b5041c5 (Avoid writing to buffer in add_excludes_from_file_1())
tried not to append '\n' at the end because the next commit
may return a buffer that does not have extra space for that.

Unfortunately it left this assignment in the loop:

  buf[i - (i && buf[i-1] == '\r')] = 0;

that can corrupt memory if "buf" is not '\n' terminated. But even if
it does not corrupt memory, the last line would not be
NULL-terminated, leading to errors later inside add_exclude().

This patch fixes it by reverting the faulty commit and make
sure "buf" is always \n terminated.

While at it, free unused memory properly.

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