git clone should ignore el-get-git-shallow-clone when using http transport.

I have set `el-get-git-shallow-clone` to `t` on my ~/.emacs.d/init.el file, as I dont want git clones to fetch full history, this however is causing the following error when trying to install `undo-tree` as it uses git http dumb transport.

```
"Cloning into 'undo-tree'...
fatal: dumb http transport does not support --depth
```

The following test proves the error.

```elisp
(setq debug-on-error t
      el-get-verbose t
      el-get-git-shallow-clone t)

(el-get 'sync 'undo-tree)
```

I'm working on fixing this, expect a pull-request with test case and fix.