Reduce the complexity of GitPushService#execute

Since today's merge of CE into EE, the complexity of the GitPushService#execute method has reached the threshold of "too high complexity" according to flog:

$ bundle exec rake flog
High complexity in GitPushService#execute. Score: 73

Build is https://gitlab.com/gitlab-org/gitlab-ee/builds/679700#down-build-trace

Attempting this one!

Reassigned to @jameslopez

mentioned in merge request !2784 (merged)

Added ~155950 label

Status changed to closed by commit 52a8d1f3

mentioned in commit df205697

Mentioned in commit pfjason/gitlab-ce@df205697

Mentioned in commit pfjason/gitlab-ce@52a8d1f3