git-debrebase: Fix snagging

git-debrebase: Fix snagging

* Rename $snags_checked to $snags_summarised and make it a counter
  of the snags we have summarised in snags_maybe_bail.
* Introduce all_snags_summarised, which uses arithmetic to see if
  we have had new snags since the most recent snags_maybe_bail,
* Use this in run_deferred_updates rather than the existing
  approach; this means we can crash if there were new snags for
  which we should have bailed.
* Make snags_maybe_bail not produce a redundant identical summary.
* Initialise counters to 0 so arithmetic does not give undef warnings.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>