whitespace: more accurate initial-indent highlighting

whitespace: more accurate initial-indent highlighting

Instead of highlighting the entire initial indent, highlight only the
problematic spaces.

In the case of an indent like ' \t \t' there may be multiple problematic
ranges, so it's easiest to emit the highlighting as we go instead of
trying rember disjoint ranges and do it all at the end.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>