add boolean diff.suppress-blank-empty config option

add boolean diff.suppress-blank-empty config option

GNU diff's --suppress-blank-empty option makes it so that diff no
longer outputs trailing white space unless the input data has it.
With this option, empty context lines are now empty also in diff -u output.
Before, they would have a single trailing space.

 * diff.c (diff_suppress_blank_empty): New global.
   (git_diff_basic_config): Set it.
   (fn_out_consume): Honor it.
 * t/t4029-diff-trailing-space.sh: New file.
 * Documentation/config.txt: Document it.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>