main: Set module names as title

If the length crosses 56 characters then the title breaks off to
multiple lines and becomes too verbose. So revert to the old message
to avoid that.

src/main.py

@@ -181,7 +181,12 @@ def commit_changes(changes: t.List[str]) -> CommittedChanges:
     log.info("Committing updates")
     body: t.Optional[str]
     if len(changes) > 1:
-        subject = "Update {} modules".format(len(changes))
+        module_names = set([i.split(":", 1)[0] for i in changes])
+        subj_string = "Update: " + ", ".join(module_names)
+        if len(subj_string) <= 56:
+            subject = subj_string
+        else:
+            subject = "Update {} modules".format(len(changes))
         body = "\n".join(changes)
         message = subject + "\n\n" + body
     else: