Note that one of the new tests panics; this is a newly discovered bug.
In Git, a commit's direct parent is allowed to also be an indirect ancestor
at the same time. `jj` currently tries to prevent this situation, but does
allow it. The correctness of `rebase -r A -d descendant_of_A` currently depends
on this jj-specific behavior; we should change that.
Cc #2600
On my Debian laptop, openssl_init() takes ~30ms to load the default CA
certificates serialized in PEM format, and the cost is added to each jj
invocation. This change saves 20s (of 50s) on my machine.
% wc -l /usr/lib/ssl/cert.pem
3517 /usr/lib/ssl/cert.pem
