gitk: Ensure tabstop setting gets restored by Cancel button

We weren't restoring the tabstop setting if the user pressed the
Cancel button in the Edit/Preferences window.  Also improved the
label for the checkbox (made it "Tab spacing" rather than the laconic
"tabstop") and moved it above the "Display nearby tags" checkbox.

Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -7419,7 +7419,7 @@ proc doprefs {} {
 	return
     }
     foreach v {maxwidth maxgraphpct diffopts showneartags showlocalchanges \
-		   limitdiffs} {
+		   limitdiffs tabstop} {
 	set oldprefs($v) [set $v]
     }
     toplevel $top
@@ -7449,14 +7449,14 @@ proc doprefs {} {
 	-font optionfont
     entry $top.diffopt -width 20 -textvariable diffopts
     grid x $top.diffoptl $top.diffopt -sticky w
+    label $top.tabstopl -text "Tab spacing" -font optionfont
+    spinbox $top.tabstop -from 1 -to 20 -width 4 -textvariable tabstop
+    grid x $top.tabstopl $top.tabstop -sticky w
     frame $top.ntag
     label $top.ntag.l -text "Display nearby tags" -font optionfont
     checkbutton $top.ntag.b -variable showneartags
     pack $top.ntag.b $top.ntag.l -side left
     grid x $top.ntag -sticky w
-    label $top.tabstopl -text "tabstop" -font optionfont
-    spinbox $top.tabstop -from 1 -to 20 -width 4 -textvariable tabstop
-    grid x $top.tabstopl $top.tabstop -sticky w
     frame $top.ldiff
     label $top.ldiff.l -text "Limit diffs to listed paths" -font optionfont
     checkbutton $top.ldiff.b -variable limitdiffs
@@ -7547,11 +7547,11 @@ proc setfg {c} {
 }
 
 proc prefscan {} {
-    global maxwidth maxgraphpct diffopts
-    global oldprefs prefstop showneartags showlocalchanges limitdiffs
+    global oldprefs prefstop
 
     foreach v {maxwidth maxgraphpct diffopts showneartags showlocalchanges \
-		   limitdiffs} {
+		   limitdiffs tabstop} {
+	global $v
 	set $v $oldprefs($v)
     }
     catch {destroy $prefstop}