Thanks for this - a minor complaint: we start error message in lower-case, so that library consumers can prepend their own error message. For example, we do:

giterr_set(GITERR_NET, "unexpected widget in the widget factory");

So that users can report:

Could not check out branch: unexpected widget in the widget factory

So - rather than bouncing this back for you to fix - I took the liberty of changing your error message. I manually merged this. Thanks for fuzzing!