Resolve Ambiguity for ties in Sort by Groups

[joeng03]

Related to #2331

Several sorting options face the issue where there is no defined order if the sort values have a tie, for example, when grouped by repo, sort the groups by author displayName (there is a possible case where an author contributed to multiple repos, and is the lexicographically largest display name among contributors of both repos.

I would propose to resolve this ambiguity in a PR, where each grouping option (by author or repo) can only sort by a fixed set of fields (for example, sort by default order might not make sense for group by author, and sort by author displayName might not make sense for group by repo). Then for sort fields that might have the same value (for example, totalCommits might be the same for several authors/repos), maybe define a default way to break ties by unique fields (for example, the default ordering of repos, or lexicogaphical order of author GitId.