Add log message at the start of the index update process

[pietroalbini]

Currently the background job for pushing a new crate to the git index doesn't log anything, making it harder to investigate outages and incidents affecting git. We should add a log message at the start of the process, showing the crate name.

[jtgeibel]

I believe this is resolved by #2231. We now log the crate name and version both before and after updating the index for all index operations (publish, yank, and unyank).