nodegit/libgit2 crashes when repository config accessed in parallel

[bpeabody]

e.g., if you run this:

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