Git status bad perfomance

[DMokhnatkin]

go-git status has too bad performance. Should be replaced by direct git call

[DMokhnatkin]

I've added optimization which allows skipping of invoke of git status if files don't need any migration. For further improvement, we can use https://github.com/libgit2/git2go instead of go-git. (but since it is a C library, it will complicate build for different platforms)