Change attribute negation marker from '!' to '-'.

At the same time, we do not want to allow arbitrary strings for
attribute names, as we are likely to want to extend the syntax
later.  Allow only alnum, dash, underscore and dot for now.

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

t/t0020-crlf.sh

@@ -216,7 +216,7 @@ test_expect_success 'apply patch --index (autocrlf=true)' '
 
 test_expect_success '.gitattributes says two is binary' '
 
-	echo "two !crlf" >.gitattributes &&
+	echo "two -crlf" >.gitattributes &&
 	rm -f tmp one dir/two &&
 	git repo-config core.autocrlf true &&
 	git read-tree --reset -u HEAD &&