Define the project whitespace policy

This establishes what the "bad" whitespaces are for this project. The rules are: - Unless otherwise specified, indent with SP that could be replaced with HT are not "bad". But SP before HT in the indent is "bad", and trailing whitespaces are "bad". - For C source files, initial indent by SP that can be replaced with HT is also "bad". - Test scripts in t/ and test vectors in its subdirectories can contain anything, so we make it unrestricted for now. Anything "bad" will be shown in WHITESPACE error indicator in diff output, and "apply --whitespace=warn" will warn about it. Signed-off-by: Junio C Hamano <gitster@pobox.com>