Include rust git logs in toolchain update PRs

[tautschnig]

Description of changes:

Fetch the commit hashes that nightly builds correspond to and construct a git log for the changes between two nightly releases. This should simplify tracking down which upstream changes may cause our builds or tests to break.

See tautschnig#11 for an example of what the result looks like.

Resolved issues:

n/a

Related RFC:

n/a

Call-outs:

n/a

Testing:

• How is this change tested? Tested in my fork (see above link).

• Is this a refactor change? No.

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• n/a Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[karkhaz]

This looks very helpful, thank you!

[zhassan-aws]

Thanks @tautschnig!