whitespace: we cannot "catch all errors known to git" anymore

whitespace: we cannot "catch all errors known to git" anymore

Traditionally, "*.txt whitespace" in .gitattributes file has been an
instruction to catch _all_ classes of whitespace errors known to git.

This has to change, however, in order to introduce "tab-in-indent" which
is inherently incompatible with "indent-with-non-tab".  As we do not want
to break configuration of existing users, add a mechanism to allow marking
selected rules to be excluded from "all rules known to git".

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