Define the project whitespace policy

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>