Conflicts like these are the fun part of using Objective-C in your plugins
The short answer to this question is: no, there is no way you can avoid these conflicts (at least in the context of code you don't control), unless you fork ObjectiveGit and rename every symbol in such a way that it does not conflict with that other version.
Alternatively, you can weakly link ObjectiveGit, and detect whether it's loaded or not. Based on that, you could try to figure out which classes/selectors/etc. that currently loaded version is using, but that gets messy real quick (and wouldn't help if their code gets loaded after yours...), so it's not really an option.
For your own C/Objective-C code, the only answer is to namespace your classes/symbols/etc. well, ideally in some way that is versioned to avoid conflicts between different versions.