CreateTag dialog: cancel should not refresh the revision grid
CommitPickerSmallControl: SetSelectedCommitHash shows message box if manually entered commit hash is not correct
FormCheckoutRevision: make similar to CreateBranch/Tag dialogs. Do not refresh grid when user hits cancel.
RevisionGrid: use StartCheckoutRevisionDialog instead of simple message box
Rename IGitUICommands.StartCheckoutBranchDialog to .StartCheckoutBranch() because this method does not always show a dialog (see form.DoDefaultActionOrShow(owner))
Merge pull request #1685 from feinstaub/topic_norefresh_on_cancel

FormCheckoutRevision: make similar to CreateBranch/Tag dialogs and use in RevisionGrid