fix: Probe executable bits by checking if we can change them

[isuffix]

After resolving jj-vcs/jj#3949 via jj-vcs/jj#7484, I realized that jj git init differs from git init and still causes issues with executable bits.

The problem is that jj git init sets up the .git/config file with core.filemode = true, despite file modes not being modifiable. Following the code, the reason is that jj uses the default file capabilities from gitoxide when initializing the repo, and gitoxide sets the capabilities by probing the filesystem. However, the probe for executable bits only tests whether the executable bit can be set when a file is created, but not whether it can be modified.

This PR changes the filemode probe to also verify that the executable bit can be modified. I tried to do so by changing as little surrounding code as possible.

The equivalent test by git itself for git init is here:
https://github.com/git/git/blob/f0ef5b6d9bcc258e4cbef93839d1b7465d5212b9/setup.c#L2367-L2379

[Byron]

Thanks a lot, great work!

I particularly like the reference to the relevant code in Git, and of course the motivation coming from JJ along with references.

It also usually makes me a little sad that GitHub de-emphasises commits to the point where PRs have all the interesting background information, but the commit has not. Maybe next time :).