diff: handle lines containing only whitespace and tabs better

When a line contains nothing but whitespace with at least one tab
and the core.whitespace config option contains blank-at-eol, the
whitespace on the line is being printed twice, once unhighlighted
(unless otherwise matched by one of the other core.whitespace values),
and a second time highlighted for blank-at-eol.

Update the leading indentation check to stop checking when it reaches
the trailing whitespace.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>