Fix commit reference short syntax decorations

Fix https://gitlab.com/gitlab-org/gitter/gitter-markdown-processor/issues/11

Previously, using a full href link was fine but the <group>/<project>@<commit sha> syntax would result in a broken link and decoration

Thanks to @peterhull90 for the downstream contribution, https://gitlab.com/gitlab-org/gitter/gitter-marked/merge_requests/11

• ✔ https://github.com/gitterHQ/sidecar/commit/ae2e261d86b8f0d2cc2fb3c92fb3a7e7c6f92617
• ✔ gitterHQ/sidecar@ae2e261d86b8f0d2cc2fb3c92fb3a7e7c6f92617
• ✔ https://gitlab.com/gitlab-org/gitlab-ce/commit/743c43e40109d0fc28a167c4b4194d69bd0f030c
• 🏁 (expected not to work) gitlab-org/gitter/webapp@6fc84297fc4ad50bb0825866c29ad58a69ae9b7e
• 🏁 (expected not to work) gitlab-org/gitlab-ce@85a1acbe50a3db74d0298fc622170f0fd000988d

Todo

• Update gitter-markdown-processor
  • ^ gitter-markdown-processor, https://gitlab.com/gitlab-org/gitter/gitter-markdown-processor/merge_requests/19
  • ^ gitter-marked, https://gitlab.com/gitlab-org/gitter/gitter-marked/merge_requests/11

cc @peterhull90