scmutil: make function name bytes in class filecache
func.__name__ returns unicodes and this leads to keyerror when we try
to do filecache[''] by passing bytes.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state