If libgit2 contained encryption code, it would fall under section 740.13(e) of the EAR, which means it doesn't need an ECCN.
However, libgit2 doesn't itself do encryption. It relies on openssl and libssh to do this, but as open-source projects, they don't require ECCN's either.
For more on this topic, see the most official answer about Mozilla code you're likely to ever get.
Also, I'm a software engineer, not a lawyer. Take this advice at your own risk.