Fix timeout error when preparing git graph.

Created by: sugarmo

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.