is there a reason not to just upgrade to 0.15?

If it compiles with both, I think this requirement is OK.

However it would probably make sense to check it continues to compile with -Z minimal-versions if we do this.

git2 depends on libgit2-sys that has a links attribute, so only one version can exist in a program. git2 versions are fragmented now, so supporting both helps: https://lib.rs/crates/git2/rev

AFAIK there aren't any real breaking changes between the two versions, so allowing both is harmless.

Cool, let's try it out