Make get_tree_size robust against git bombs

[jorendorff]

Description

For pathological repositories like git bombs with deeply nested tree structures, Rugged's count_recursive method can hang indefinitely even with a file limit, because it has to visit every tree object to find and count the blobs. See libgit2/rugged#995.

That causes RuggedRepository#get_tree_size to hang, so git-linguist stats can hang.

This PR works around that issue by using a custom count method that also applies a tree limit, so that it terminates promptly regardless of repository structure. The return value is still the blob count for normal repos, preserving existing behavior.

Checklist:

• I am adding new or changing current functionality
  • I have added or updated the tests for the new or changed functionality.

[carlosmn]

This reimplements count_recursive but with the tree limit and the trick in the implementation to return the limit if we have too many trees. I think ideally the tree limit would be different and we could also just return 0 or some value that means unknown if we do see too many trees, but I don't know where all this ends up getting used.

        def get_tree_size(commit_id, limit)
          tree_count = 0
          blob_count = 0
          get_tree(commit_id).walk(:preorder) do |root, entry|
            if blob_count >= limit || tree_count >= limit
              blob_count = limit # If we have too many trees we return the limit
              raise StopIteration
            end

            case entry[:type]
            when :blob
              blob_count += 1
            when :tree
              tree_count += 1
            end

            true # go into a tree if that's what we were given
          end

          blob_count
        rescue StopIteration
          blob_count
        end

[jorendorff]

I ended up with this. This is now ready for review.