Fix automatic close of Rebase dialog when pull.rebase is set

Merged username-removed-360247 requested to merge engycz/tortoisegit:fix_close into master

When current branch is forward from upstream, send BN_CLICKED to Abort button to close the dialog.

No need to call m_CommitList.GetItemCount() on line 577, m_CommitList is cleared on line 573 and has zero size.

Repo to test repo1.zip

With preview 1.8.16.4 the Rebase dialog remains open when pull.rebase=true is set