Merge branch 'pw/3.0-commentchar-auto-deprecation'

"core.commentChar=auto" that attempts to dynamically pick a
suitable comment character is non-workable, as it is too much
trouble to support for little benefit, and is marked as deprecated.

* pw/3.0-commentchar-auto-deprecation:
  commit: print advice when core.commentString=auto
  config: warn on core.commentString=auto
  breaking-changes: deprecate support for core.commentString=auto

builtin/rebase.c

@@ -1242,6 +1242,9 @@ int cmd_rebase(int argc,
 					 builtin_rebase_usage,
 					 builtin_rebase_options);
 
+#ifndef WITH_BREAKING_CHANGES
+	warn_on_auto_comment_char = true;
+#endif /* !WITH_BREAKING_CHANGES */
 	prepare_repo_settings(the_repository);
 	the_repository->settings.command_requires_full_index = 0;