apply --whitespace=fix: fix tab-in-indent

apply --whitespace=fix: fix tab-in-indent

When the whitespace rule tab-in-indent is enabled, apply --whitespace=fix
replaces tabs by the appropriate amount of blanks. The code used
"dst->len % 8" as the criterion to stop adding blanks. But it forgot that
dst holds more than just the current line. Consequently, the modulus was
computed correctly only for the first added line, but not for the second
and subsequent lines. Fix it.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>