fix: preserve flow when rebinning

src/boost_histogram/histogram.py

@@ -975,18 +975,18 @@ def __getitem__(self: H, index: IndexingExpr) -> H | float | Accumulator:
 
                     new_reduced = reduced.__class__(axes)
                     new_view = new_reduced.view(flow=True)
-
-                    j = 1
+                    j = 0
+                    groups = [1] + groups + [1]  # pad for flow bins
                     for new_j, group in enumerate(groups):
                         for _ in range(group):
                             pos = [slice(None)] * (i)
                             if new_view.dtype.names:
                                 for field in new_view.dtype.names:
-                                    new_view[(*pos, new_j + 1, ...)][field] += (
+                                    new_view[(*pos, new_j, ...)][field] += (
                                         reduced_view[(*pos, j, ...)][field]
                                     )
                             else:
-                                new_view[(*pos, new_j + 1, ...)] += reduced_view[
+                                new_view[(*pos, new_j, ...)] += reduced_view[
                                     (*pos, j, ...)
                                 ]
                             j += 1