Enable triagebot no-merges check

This configuration will exclude rollup PRs and subtree/submodule sync
PRs from merge commit detection, and it will post the default warning
message and add the `has-merge-commits` and `S-waiting-on-author` labels
when merge commits are detected on other PRs.

The eventual vision is to have bors refuse to merge if the
`has-merge-commits` label is present. A reviewer can still
force the merge by removing that label if they so wish.

triagebot.toml

@@ -310,6 +310,10 @@ message_on_add = """\
 Issue #{number} "{title}" has been added.
 """
 
+[no-merges]
+exclude_labels = ["rollup", "sync"]
+labels = ["has-merge-commits", "S-waiting-on-author"]
+
 [github-releases]
 format = "rustc"
 project-name = "Rust"