Pruning of commits which become empty can result in a variety of
topology changes: a merge may have lost all its ancestors corresponding
to one of (or more) of its parents, a merge may end up merging a commit
with itself, or a merge may end up merging a commit with its own
ancestor. Merging a commit with itself makes no sense, so we'd rather
prune down to one parent and hopefully prune the merge commit, but we do
need to worry about whether the are changes in the commit and whether
the original merge commit also merged something with itself. We have
similar cases for dealing with a merge of some commit with its own
ancestor: if the original topology did the same, or the merge commit has
additional file changes, then we cannot remove the commit. But,
otherwise, the commit can be pruned.
Add testcases covering the variety of changes that can occur to make
sure we get them right.
Signed-off-by: Elijah Newren <newren@gmail.com>
