Improved hgweb test. The hgweb fix in 069b29656401 aimed at restoring the "back" link in hgweb's filelog that has been lost in 62e0bb41e682. However, the previous version of this test ran the filelog command on a file with only a single filelog entry. In this case, the previous hgweb version did not exhibit the bug. The error condition is now correctly tested with a filelog of 2 entries.