[Utils] Add URL formatting for revert_checker

This lets the revert_checker.py get called with the -u option, which
formats the revert and reverted SHAs into handy URLs which point to the
LLVM reviews associated with those SHAs. This is useful for viewers to
look quickly at the changes made by SHAs that were potentially reverted.

Differential Revision: https://reviews.llvm.org/D122772