Fix hgweb.filediff
The third argument to hgweb.diff should be a list, not a string.
This fixes a bug found by xorAxAx, where hgweb would also show the diff
for "README" when you ask for the diff for "Demo/cgi/README".
changeset: 0:9426b370c206tag: tipuser: My Name <myname@example.com>date: Mon Jan 12 13:46:40 1970 +0000summary: commit-1