whitespace: replumb ws_fix_copy to take a strbuf *dst instead of char *dst

whitespace: replumb ws_fix_copy to take a strbuf *dst instead of char *dst

To implement --whitespace=fix for tab-in-indent, we have to allow for the
possibility that whitespace can increase in size when it is fixed, expanding
tabs to to multiple spaces in the initial indent.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>