Use ls-files instead of ls-tree to run whitespace tests.

This allows us to check uncommitted files (while still ignoring
ignored files).

Anything that should go into the repo that is not specifically
excluded from testing will be tested.

Note that this also removed the -f test since only files are returned
from ls-files.

t/whitespace.t

@@ -11,8 +11,8 @@ BEGIN {
     push(@exempted, glob("doc/xml2rfc/*.xsl"));
     my %exempted_hash = map { $_ => 1 } @exempted;
 
-    my @stuff = split /\0/, `git ls-tree -r -z --name-only HEAD`;
-    @files = grep { -f $_ && ! $exempted_hash{$_} } @stuff;
+    my @stuff = split /\0/, `git ls-files -z -c -m -o --exclude-standard`;
+    @files = grep { ! $exempted_hash{$_} } @stuff;
 
     # We won't find any files if git isn't installed.  If git isn't
     # installed, they're probably not doing any useful development, or