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

plus some minor changes:
- Create Tag dialog: cancel now does not refresh the revision grid
- CommitPickerSmallControl: SetSelectedCommitHash now shows message box if manually entered commit hash is not correct
- Rename IGitUICommands.StartCheckoutBranchDialog to .StartCheckoutBranch() because this method does not always show a dialog