bpo-34588: Fix an off-by-one error in traceback formatting. (GH-9077)

The recursive frame pruning code always undercounted the number of elided frames
by one. That is, in the "[Previous line repeated N more times]" message, N would
always be one too few. Near the recursive pruning cutoff, one frame could be
silently dropped. That situation is demonstrated in the OP of the bug report.

The fix is to start the identical frame counter at 1.