convert: use bytes for value in extra dict
The keys and values of the extra dict are expected to be bytes. Before this
fix, there was a crash in mercurial.changelog.encodeextra().
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint