handle write-collisions and permissions

[Byron]

Tasks

• reproduce collisions
• fix collisions
• adjust permission bits to be similar to what git does after creating new loose objects

Fixes #819

[Byron]

Permissions are quite complex in detail, but are now fixed to the point where they look similar to what git produces with default settings.

On the bright side, gitoxide will now do what git does to avoid writing duplicate loose objects to disk, which should prevent typical collisions on windows all by itself. If the object DB is used directly, it will now ignore access-denied errors on windows, assuming that there is no actual hash collision, but an issue with the filesystem. This is equivalent to the unix implementation which would also overwrite possibly existing files by renaming.

[jpgrayson]

Thanks, @Byron. This is great! I really appreciate it.

[Byron]

I really appreciate it too :) - when investigating this issue I noticed a lot of rw objects which were clearly written by gitoxide, and I also want to see less of that. Furthermore I'd hope that overall write speeds are now increased by preventing duplication, let's see if it makes a difference for stg workloads.

Please note that the implementation might well be slower now as the additional chmod call is something git doesn't have to do. I checked with the tempfile crate if this is something that could change.