It's up to the implementation, of course, but I believe 100 is reserved for exact equality, as detected by OID sameness.

Note that we have an OID for the tree entry and an OID for the index entry, but we do not have an OID for the workdir entry (since, of course, we'd have to calculate it.)  We calculate the OID when doing exact rename-only matches, otherwise we do not bother, since 99% similarity is about the same as 100% similarity when we're not doing exact-only matches.

So.  No.  I wouldn't expect that with the current libgit2 implementation.  I pondered making a change for that in libgit2 (calculating the OID when we read the similarity), but it seemed like a non-issue.