Reworking git module

[Nhoya]

At the moment the git module is limited to git API or memory space.
This can lead to 2 problems:

• The git website API are too limited and will block the mail fetching + one request per 100 commits is not so good
• Since the plain search is done cloning the entire repo in memory is not a really good thing with big repos

The possible solution was to implement the equivalent for git clone -n but is not currently supported from go-git and i don't want to depend from a wrapper like https://github.com/libgit2/git2go (pure go is always better), less dependencies, less problems

So I'm waiting the implementation of src-d/go-git/pull/721 to proceed with the reworking

The new module should be way faster and less resource hungry

[Nhoya]

Update: the PR was merged, waiting for the new release