clean.c: use designated initializer

clean.c: use designated initializer

This is another test balloon to see if we get complaints from people
whose compilers do not support designated initializer for arrays.

The use of the feature is not all that interesting for cases like
the one this patch touches, where the initialized elements of the
array is dense, but it would be nice if we can use the feature to
initialize an array that has elements initialized to interesting
values only sparsely.

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