gitaly_client: Reimplement `#between` via `#list_commits`

The CommitsBetween RPC call has been deprecated by Gitaly in favor of
ListCommits, which is a much more flexible alternative which allows the
caller to specify a set of revisions and various other parameters.
Reimplement `#between` on top of `#list_commits` such that Gitaly can
eventually drop CommitsBetween.

Note that instead of using the range-notation "fromrev..torev", we use
its equivalent spelling "^fromrev torev". See also the gitrevisions(7)
man page.

config/feature_flags/development/between_commits_via_list_commits.yml

+---
+name: between_commits_via_list_commits
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74273
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/345458
+milestone: '14.5'
+type: development
+group: group::gitaly
+default_enabled: false

lib/gitlab/gitaly_client/commit_service.rb

@@ -205,6 +205,8 @@ module Gitlab
       end
 
       def between(from, to)
+        return list_commits(["^" + from, to], reverse: true) if Feature.enabled?(:between_commits_via_list_commits)
+
         request = Gitaly::CommitsBetweenRequest.new(
           repository: @gitaly_repo,
           from: from,

spec/lib/gitlab/gitaly_client/commit_service_spec.rb

@@ -112,6 +112,28 @@ RSpec.describe Gitlab::GitalyClient::CommitService do
     let(:from) { 'master' }
     let(:to) { Gitlab::Git::EMPTY_TREE_ID }
 
+    context 'with between_commits_via_list_commits enabled' do
+      before do
+        stub_feature_flags(between_commits_via_list_commits: true)
+      end
+
+      it 'sends an RPC request' do
+        request = Gitaly::ListCommitsRequest.new(
+          repository: repository_message, revisions: ["^" + from, to], reverse: true
+        )
+
+        expect_any_instance_of(Gitaly::CommitService::Stub).to receive(:list_commits)
+          .with(request, kind_of(Hash)).and_return([])
+
+        described_class.new(repository).between(from, to)
+      end
+    end
+
+    context 'with between_commits_via_list_commits disabled' do
+      before do
+        stub_feature_flags(between_commits_via_list_commits: false)
+      end
+
       it 'sends an RPC request' do
         request = Gitaly::CommitsBetweenRequest.new(
           repository: repository_message, from: from, to: to
@@ -123,6 +145,7 @@ RSpec.describe Gitlab::GitalyClient::CommitService do
         described_class.new(repository).between(from, to)
       end
     end
+  end
 
   describe '#diff_stats' do
     let(:left_commit_id) { 'master' }