Then common Git commands (diff, log, bisect, etc.) will behave as if the oldest commit on the "central" branch (e18f9a3) is the newest commit on the "cvs" branch (3ec464b), and you can traverse history accordingly.
Note that the GitHub website doesn't support the replacement ref, presumably because it depends on libgit2, which doesn't support replacement refs per [https://github.com/libgit2/libgit2/issues/1730 issue 1730]. So you can't traverse the combined history of both branches transparently on the GitHub website.
=== Using mozilla/gecko With try/inbound ===