darcs patch: Try to replace the index file atomically.

This is only locally tested in a copy of the archive. Sun Mar 2 19:42:01 GMT 2008 Duncan Coutts * Try to replace the index file atomically. Using gzip > file means the file is created/replaced at the begining of the procedure, so for the period until it finishes the index file is in fact invalid. So re-generate the index into a temp file and then atomically rename. This does not make the regeneration totally atomic since while it is being generated, other files could be being added to the archive. BTW, could we set the 'darcs send' email address for the hackage-scripts repo so it works automagically. Would it also make sense for changes in the hackage-scripts to be sent to cabal-devel, like for the Cabal and cabal-install repos? Duncan