git-diff: complain about >=8 consecutive spaces in initial indent

git-diff: complain about >=8 consecutive spaces in initial indent

This introduces a new whitespace error type, "indent-with-non-tab".
The error is about starting a line with 8 or more SP, instead of
indenting it with a HT.

This is not enabled by default, as some projects employ an
indenting policy to use only SPs and no HTs.

The kernel folks and git contributors may want to enable this
detection with:

	[core]
		whitespace = indent-with-non-tab

Signed-off-by: Junio C Hamano <gitster@pobox.com>