gitk: Use a spinbox for setting tabstop settings

The tabstop must be a smallish positive integer, and a spinbox is the
accepted UI control to accomplish this limiting rather than the text
entry box previously used.

Signed-off-by: Mark Levedahl <mdl123@verizon.net>

gitk

@@ -6972,7 +6972,7 @@ proc doprefs {} {
     pack $top.ntag.b $top.ntag.l -side left
     grid x $top.ntag -sticky w
     label $top.tabstopl -text "tabstop" -font optionfont
-    entry $top.tabstop -width 10 -textvariable tabstop
+    spinbox $top.tabstop -from 1 -to 20 -width 4 -textvariable tabstop
     grid x $top.tabstopl $top.tabstop -sticky w
 
     label $top.cdisp -text "Colors: press to choose"