previously, `jj diff` would show the full contents of binary files such as images.
after this change, it instead shows "(binary)". it still shows the filename and metadata so that
users can open the file in the viewer of their choce.
future work could involve showing binary files as Sixel or similar; finding a way to compare large
non-binary files without filling up the screen; or extending the data backends to avoid having to
read the whole file contents into memory.
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
