~/.config/git/gitk overritten every time

according to https://stackoverflow.com/questions/8221877/gitk-setting-ignore-space-change-option-to-be-true-by-default ,
I added
set ignorespace 1 # the added line
into ~/.config/git/gitk file,
however, this file is overwritten every time when gitk exit.
and the added line is removed.

It seems that it's not necessary recreate ~/.config/git/gitk , please advice, and
Thanks !

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gitk 1:2.17.1-1ubuntu0.5
ProcVersionSignature: Ubuntu 4.15.0-47.50-generic 4.15.18
Uname: Linux 4.15.0-47-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.9-0ubuntu7.9
Architecture: amd64
CurrentDesktop: Unity:Unity7:ubuntu
Date: Sun Jan 26 12:03:46 2020
InstallationDate: Installed on 2019-01-30 (361 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
PackageArchitecture: all
SourcePackage: git
UpgradeStatus: No upgrade log present (probably fresh install)