apply --whitespace=fix: fix handling of blank lines at the eof

apply --whitespace=fix: fix handling of blank lines at the eof

b94f2ed (builtin-apply.c: make it more line oriented, 2008-01-26) broke
the logic used to detect if a hunk adds blank lines at the end of the
file.  With the new code after that commit:

 - img holds the contents of the file that the hunk is being applied to;

 - preimage has the lines the hunk expects to be in img; and

 - postimage has the lines the hunk wants to update the part in img that
   corresponds to preimage with.

and we need to compare if the last line of preimage (not postimage)
matches the last line of img to see if the hunk applies at the end of the
file.

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