git-gui: Refactor diff pane popup support for future improvements

The current popup_diff_menu procedure is somewhat messy as it has a
few duplications of the same logic in each of the different legs of
the routine.  We can simplify these by setting a few state variables
in the different legs.

No functional change, just a cleanup to make it easier to implement
future functional changes within this block.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2432,20 +2432,19 @@ proc popup_diff_menu {ctxm x y X Y} {
 	set ::cursorX $x
 	set ::cursorY $y
 	if {$::ui_index eq $::current_diff_side} {
-		$ctxm entryconf $::ui_diff_applyhunk \
-			-state normal \
-			-label {Unstage Hunk From Commit}
-	} elseif {$current_diff_path eq {}
-		|| ![info exists file_states($current_diff_path)]
-		|| {_O} eq [lindex $file_states($current_diff_path) 0]} {
-		$ctxm entryconf $::ui_diff_applyhunk \
-			-state disabled \
-			-label {Stage Hunk For Commit}
+		set s normal
+		set l "Unstage Hunk From Commit"
 	} else {
-		$ctxm entryconf $::ui_diff_applyhunk \
-			-state normal \
-			-label {Stage Hunk For Commit}
+		if {$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"
 	}
+	$ctxm entryconf $::ui_diff_applyhunk -state $s -label $l
 	tk_popup $ctxm $X $Y
 }
 bind_button3 $ui_diff [list popup_diff_menu $ctxm %x %y %X %Y]