gitk: Persist config for ignore space and diff mode

[lostindark]

Save the user's choice for "Ignore space change: and diff mode so they don't have to adjust these options every time they open Gitk if the defaults don't work for them.

[j6t]

Thank you.

Personally, I am not a fan of this change, because the default settings work for me and I would be surprised if I don't see the expected diff after opening Gitk just because "Ignore space change" has survived the session. I do use the setting, but it is ingrained in my head that it not persistent. Same goes for the diff mode setting.

You might want to submit a patch (MUA hints) to the Git mailing list to get more opinions.

As for the change itself: You moved the initialization, but forgot the surrounding [mc ... ]. Some of the text in this PR could be reused in the commit message. But please do not write "This change is...", but write in imperative tone. And if necessary, describe the status quo (before the change) in present tense.