whitespace: add tab-in-indent error class

whitespace: add tab-in-indent error class

Some projects and languages use coding style where no tab character is used to
indent the lines.

This only adds support and documentation for "apply --whitespace=warn" and
"diff --check"; later patches add "apply --whitespace=fix" and tests.

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