Fix maxConcurrentGitOperations

[bakkerthehacker]

Somewhere in the move to a promise based git system, ungit stopped using this config option.

This changes adds the maxConcurrentGitOperations option back. This change is quite important to me as several repositories only work well with maxConcurrentGitOperations = 1. Without it, ungit complains about index.lock files

[jung-kim]

I like the thought but why are we bringing in async lib? There is no need for this.

I see, async lib is parallelizing calls... Hmm... I'd rather solve this problem with bluebird. And isn't this only a problem if multi git process are running on a same git repo?

[jung-kim]

Merging as there is no clear and easy alternate way to do this in bluebird. Although there are some other bluebird plugins we could use it hardly seems worth it.