Fix Git repository for report (use root directory).

The previous code was expecting `coveralls` to be executed from the source directory. However, we may do something like:

```sh
mkdir -p /tmp/build && cd /tmp/build
cmake /path/to/project
make && make test
coveralls -b . -r /path/to/project
```

In this case, the `build` directory is not in the Git project, leading to the following error:
```text
fatal: Not a git repository (or any parent up to mount point /tmp)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
```
Thus, we use the `root` directory when looking for the Git repository.