more info is gathered on metaupdate in background indexer

The background indexer now gathers info on contributors and modification
dates from the changelog when adding the missing meta info.

A new io_grep function was added which might be useful for other parts in
the Wiki as well.

darcs-hash:20060511191450-7ad00-baba1b48ea03b823c88a480862c612316f159b5a.gz