git-gui: Properly set the state of "Stage/Unstage Hunk" action

Today I found yet another way for the "Stage Hunk" and "Unstage
Hunk" context menu actions to leave the wrong state enabled in
the UI.  The problem this time was that I connected the state
determination to the value of $::current_diff_side (the side the
diff is from).  When the user was last looking at a diff from the
index side and unstages everything the diff panel goes empty, but
the action stayed enabled as we always assumed unstaging was a
valid action.

This change moves the logic for determining when the action is
enabled away from the individual side selection, as they really
are two unrelated concepts.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2441,20 +2441,17 @@ proc popup_diff_menu {ctxm x y X Y} {
 	set ::cursorX $x
 	set ::cursorY $y
 	if {$::ui_index eq $::current_diff_side} {
-		set s normal
 		set l "Unstage Hunk From Commit"
 	} else {
-		if {$current_diff_path eq {}
-			|| ![info exists file_states($current_diff_path)]
-			|| {_O} eq [lindex $file_states($current_diff_path) 0]} {
-			set s disabled
-		} else {
-			set s normal
-		}
 		set l "Stage Hunk For Commit"
 	}
-	if {$::is_3way_diff} {
+	if {$::is_3way_diff
+		|| $current_diff_path eq {}
+		|| ![info exists file_states($current_diff_path)]
+		|| {_O} eq [lindex $file_states($current_diff_path) 0]} {
 		set s disabled
+	} else {
+		set s normal
 	}
 	$ctxm entryconf $::ui_diff_applyhunk -state $s -label $l
 	tk_popup $ctxm $X $Y