Rollup merge of #46539 - oli-obk:cargo_lock, r=alexcrichton

Do not automatically merge `Cargo.lock`

It essentially never does what it's supposed to and often leaves the `Cargo.lock` in a state where it needs manual adjustments or resetting to master/yourbranch. With this setting git will always choose your `Cargo.lock` over the one from master and report a merge error (if both master and your branch touched `Cargo.lock`). Now you can run `./x.py help` and it should normally update the `Cargo.lock` to be one that has everything needed by master and your branch.