gitk: Ensure tabstop setting gets restored by Cancel button

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>