installer: properly save fast-forward only choice

6ab14d9 (installer: allow configuring the default behavior of `git pull`,
2020-05-19) introduced radio buttons to select how `git pull` behaves by default.
The `fast-forward only` option currently gets wrongly saved as `merge`, though.

Signed-off-by: Matthias Aßhauer <[email protected]>

Author: rimrul

installer/install.iss

@@ -3130,7 +3130,7 @@ begin
     Data:='Merge';
     if RdbGitPullBehavior[GP_GitPullRebase].Checked then begin
         Data:='Rebase'
-    end else if RdbGitPullBehavior[GP_GitPullRebase].Checked then begin
+    end else if RdbGitPullBehavior[GP_GitPullFFOnly].Checked then begin
         Data:='FFOnly'
     end;
     RecordChoice(PreviousDataKey,'Git Pull Behavior Option',Data);