Unnecessary getSubmodulePaths

[cblp]

I'm trying to solve one particular problem — to get a branch name for a repository inside a given path.

I'm doing

var GitUtils = require('git-utils')
console.log(GitUtils.open(path).getShortHead())

But open performs too slow on sshfs. It takes about 5 or 6 seconds to getSubmodulePaths. The other code takes about 10 ms.

I suggest to delay openSubmodules for the time when it will be needed, and to not call it in open.

[cblp]

Converted to PR #52