bootstrap update notifications: only show what's new

[RalfJung]

Bootstrap now remembers which "update warnings" it already showed me, and so when I run it the second time it doesn't keep repeating the same messages. That's great. :)

However, when something does change, it prints more than I would expect:

There have been changes to x.py since you last updated:
  [WARNING] The "codegen"/"llvm" profile has been removed and replaced with "compiler", use it instead for the same behavior.
    - PR Link https://github.com/rust-lang/rust/pull/121278
  [INFO] `x install` now skips providing tarball sources (under 'build/dist' path) to speed up the installation process.
    - PR Link https://github.com/rust-lang/rust/pull/118724
  [INFO] A new `boostrap-cache-path` option has been introduced which can be utilized to modify the cache path for bootstrap.
    - PR Link https://github.com/rust-lang/rust/pull/121976
  [INFO] a new `target.*.runner` option is available to specify a wrapper executable required to run tests for a target
    - PR Link https://github.com/rust-lang/rust/pull/122108
  [INFO] New option `rust.llvm-bitcode-linker` that will build the llvm-bitcode-linker.
    - PR Link https://github.com/rust-lang/rust/pull/117458
NOTE: to silence this warning, update `config.toml` to use `change-id = 117458` instead

At least the first 2 I have already seen, but somehow it keeps repeating them every time a new item is added to the list.

Is it possible that when computing which thing to show, it uses the value in the config.toml rather than the value in the "last things I showed" file? If so, would it be possible to only show what's new since the last time something was shown? That way I don't have to re-read the same update notifications over and over again, and the list does not keep growing indefinitely. :)

Cc @onur-ozkan