FormRebase: select first parent which is a better default selection

due to how rebase onto is working
(the selected revision is excluded from the rebase --as it is the "base")

So if the user want to rebase only one commit (which is not so rare),
he only has to validate the selection...

GitUI/CommandsDialogs/FormRebase.cs

@@ -386,7 +386,9 @@ private void btnChooseFromRevision_Click(object sender, EventArgs e)
             try
             {
                 AppSettings.ShowStashes = false;
-                using FormChooseCommit chooseForm = new(UICommands, txtFrom.Text, showCurrentBranchOnly: true);
+                ObjectId firstParent = UICommands.GitModule.RevParse("HEAD~");
+                string preSelectedCommit = !string.IsNullOrWhiteSpace(txtFrom.Text) ? txtFrom.Text : firstParent?.ToString() ?? string.Empty;
+                using FormChooseCommit chooseForm = new(UICommands, preSelectedCommit, showCurrentBranchOnly: true);
                 if (chooseForm.ShowDialog(this) == DialogResult.OK && chooseForm.SelectedRevision is not null)
                 {
                     txtFrom.Text = chooseForm.SelectedRevision.ObjectId.ToShortString();