Diff::CommitDiff fails on very large diffs

As reported by @yhl3692319 in #209 (comment 29401212).

I pushed a commit with a new_file.c(the size of this file is 5M) or 1000+ diff files, when I click this commit show, grpc went wrong.

GRPC::Core::CallError (call#run_batch failed somehow):
  lib/gitlab/git/diff_collection.rb:93:in `each_with_index'
  lib/gitlab/git/diff_collection.rb:93:in `each_patch'
  lib/gitlab/git/diff_collection.rb:34:in `block in each'
  lib/gitlab/gitaly_client.rb:73:in `block in migrate'
  lib/gitlab/metrics.rb:93:in `measure'
  lib/gitlab/gitaly_client.rb:72:in `migrate'
  lib/gitlab/git/diff_collection.rb:33:in `each'
  lib/gitlab/git/diff_collection.rb:63:in `each_with_index'

Worth investigating whether this is related to https://gitlab.com/gitlab-org/gitlab-ce/issues/31991 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11244 in some way?

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