apply --whitespace: configuration option.

The new configuration option apply.whitespace can take one of
"warn", "error", "error-all", or "strip".  When git-apply is run
to apply the patch to the index, they are used as the default
value if there is no command line --whitespace option.

Andrew can now tell people who feed him git trees to update to
this version and say:

git repo-config apply.whitespace error

Signed-off-by: Junio C Hamano <junkio@cox.net>