patchgitk: Ensure that "Reset branch" menu entry is enabled

> Consider this sequence of events:
> 
> 1. Detach HEAD and fire up gitk
> 2. Call the context menu on some commit. Notice that the last menu entry
>    says "Detached HEAD: can't reset" and it is disabled.
> 3. Now checkout some regular branch (e.g. 'master') using the context menu.
> 4. Call the context menu again on some commit.
> 
> Previously, at this point the last menu entry said "Reset master branch
> to here", but it was still disabled. With this fix it is now enabled again.

> 
> Signed-off-by: Johannes Sixt <j6t@kdbg.org>
> ---
>  gitk |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/gitk b/gitk
> index f7f1776..6c4022a 100755
> --- a/gitk
> +++ b/gitk
> @@ -7950,7 +7950,7 @@ proc rowmenu {x y id} {
>      if {$id ne $nullid && $id ne $nullid2} {
>  	set menu $rowctxmenu
>  	if {$mainhead ne {}} {
> -	    $menu entryconfigure 7 -label [mc "Reset %s branch to here" $mainhead]
> +	    $menu entryconfigure 7 -label [mc "Reset %s branch to here" $mainhead] -state normal
>  	} else {
>  	    $menu entryconfigure 7 -label [mc "Detached head: can't reset" $mainhead] -state disabled
>  	}