add_excludes_from_file: clarify the bom skipping logic

add_excludes_from_file: clarify the bom skipping logic

Even though the previous step shifts where the "entry" begins, we
still iterate over the original buf[], which may begin with the
UTF-8 BOM we are supposed to be skipping.  At the end of the first
line, the code grabs the contents of it starting at "entry", so
there is nothing wrong per-se, but the logic looks really confused.

Instead, move the buf pointer and shrink its size, to truly
pretend that UTF-8 BOM did not exist in the input.

Signed-off-by: Junio C Hamano <gitster@pobox.com>