Explicitly call #to_a to avoid potential GC bug

We've seen a small number of seg faults that look to be coming from this
method. One candidate for cause is a known issue in Ruby's GC with
flat_map (https://bugs.ruby-lang.org/issues/18140) because the
concatenated array can be GC'd prematurely. By calling #to_a explicitly,
we insert it into the Ruby stack so that GC can recognize it and keep it
around.

There is a fix and backport for this in Ruby, but it may be a while
before they're released.

h/t to @stanhu for identifying this as a possible root cause and
supplying a solution.

lib/gitlab/gitaly_client/commit_service.rb

@@ -212,7 +212,7 @@ module Gitlab
         )
 
         response = GitalyClient.call(@repository.storage, :diff_service, :diff_stats, request, timeout: GitalyClient.medium_timeout)
-        response.flat_map(&:stats)
+        response.flat_map { |rsp| rsp.stats.to_a }
       end
 
       def find_changed_paths(commits)