[Utils] Add URL formatting for revert_checker

[CrystalSplitter]

Summary

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.

[george.burgess.iv]

LGTM, thanks!

Do you have commit access, or should I land this for you?

[CrystalSplitter]

I do not appear to have commit access. I don't see any buttons to do so, and this account is 50 minutes old--but I could just be ignorant. :)

llvm/utils/revert_checker.py

parser = argparse.ArgumentParser(
       description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter)
   parser.add_argument(
       'base_ref', help='Git ref or sha to check for reverts around.')
   parser.add_argument(
       '-C', '--git_dir', default='.', help='Git directory to use.')
   parser.add_argument(
       'root', nargs='+', help='Root(s) to search for commits from.')
   parser.add_argument('--debug', action='store_true')
+  parser.add_argument(
+      '-u', '--review_url', action='store_true',
+      help='Format SHAs as llvm review URLs')
   opts = parser.parse_args()
 
   logging.basicConfig(
       format='%(asctime)s: %(levelname)s: %(filename)s:%(lineno)d: %(message)s',
       level=logging.DEBUG if opts.debug else logging.INFO,
   )
 
   # `root`s can have related history, so we want to filter duplicate commits
   # out. The overwhelmingly common case is also to have one root, and it's way
   # easier to reason about output that comes in an order that's meaningful to
   # git.
   seen_reverts = set()
   all_reverts = []
   for root in opts.root:
     for revert in find_reverts(opts.git_dir, opts.base_ref, root):
       if revert not in seen_reverts:
         seen_reverts.add(revert)
         all_reverts.append(revert)
 
   for revert in all_reverts:
-    print(f'{revert.sha} claims to revert {revert.reverted_sha}')
+    sha_fmt = (f'https://reviews.llvm.org/rG{revert.sha}'
+               if opts.review_url else revert.sha)
+    reverted_sha_fmt = (f'https://reviews.llvm.org/rG{revert.reverted_sha}'
+                        if opts.review_url else revert.reverted_sha)
+    print(f'{sha_fmt} claims to revert {reverted_sha_fmt}')
 
 
 if __name__ == '__main__':
   _main()