Use Go-Git for the FindCommit RPC

Zeger-Jan van de Wegrequested to merge
zj-go-git-find-commit into master

Using go-git for the FindCommit RPC is between 5 and 18 times faster.

This branch got used in CI in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18704, pipeline link

Edited by GitLab Release Tools Bot

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