Fix timeout error when preparing git graph.

**gitlab_git** use 30s as the default timeout limit. But, if **Gitlab** is running on a slow machine, 30s is still not long enough to load and display large project's graph. (My machine takes 70s to do this)

Changing timeout setting in `gitlab.yml` doesn't help anying, it's weird. It should change that timeout setting in graph too.

I found that there is already an interface in `gitlab_git` to change timeout setting, then I use it, then my problem resolved. Why **Gitlab** did not do this before?

I hope this PR can be merged, this can really help many people.