Add new diff stat rpc

Issue: gitlab#227572 (closed)

Create a new RPC to return the changes between an array of commits

Comparing diff-tree --numstat vs diff --numstat on the gitlab repo:

time (for i in {1..50}
do
  git diff-tree --numstat HEAD~${i} HEAD~$((i+1));
done)

0.54s user 0.39s system 88% cpu 1.057 total

time (for i in {1..50}
do
  git diff --numstat HEAD~${i}..HEAD~$((i+1)) | cat;
done)

0.32s user 0.34s system 98% cpu 0.666 total

time (git rev-list --first-parent HEAD~50..HEAD | git diff-tree --numstat --stdin -m -c --no-commit-id)

0.11s user 0.04s system 103% cpu 0.153 total