nodegit/libgit2 crashes when repository config accessed in parallel

e.g., if you run this:
```JavaScript
const config = repo.config();
const value = config.getString("section.value");
```
many times in parallel, you will eventually crash with:

```
Assertion failed: (c == '['), function parse_section_header, file ../vendor/libgit2/src/config_file.c, line 1126.
error: git-meta died of signal 6
```