Have a WPF treeview stretch to fit its container

 <Grid Margin="5">
     <Grid.Resources>
         <HierarchicalDataTemplate DataType="{x:Type viewmodels:GistViewModel}" ItemsSource="{Binding Path=GistFiles}">
             <TextBlock Text="{Binding GistFiles, Converter={StaticResource GistFilesToFirstFilenameConverter}}"></TextBlock>
         </HierarchicalDataTemplate>
         <DataTemplate DataType="{x:Type viewmodels:GistFileViewModel}">
             <TextBlock Text="{Binding Filename}"></TextBlock>
         </DataTemplate>
     </Grid.Resources>
     <TreeView x:Name="GistsTV" Grid.Column="0" ItemsSource="{Binding Gists}">
     </TreeView>            
 </Grid>

 <Grid Margin="5">
     <Grid.Resources>
         <HierarchicalDataTemplate DataType="{x:Type viewmodels:GistViewModel}" ItemsSource="{Binding Path=GistFiles}">
             <TextBlock Text="{Binding GistFiles, Converter={StaticResource GistFilesToFirstFilenameConverter}}"></TextBlock>
         </HierarchicalDataTemplate>
         <DataTemplate DataType="{x:Type viewmodels:GistFileViewModel}">
             <TextBlock Text="{Binding Filename}"></TextBlock>
         </DataTemplate>
     </Grid.Resources>
     <TreeView x:Name="GistsTV" Grid.Column="0" ItemsSource="{Binding Gists}">
     </TreeView>            
 </Grid>