repo: more arg checking for --standalone-manifest re-inits

`repo init` doesn't do anything on re-init when the checkout has
been initialized using --standalone manifest. Rather than let the
tool run through its existing flows (which happen to noop), check
the args and explicitly quit if a bare `repo init` is run on a
standalone checkout.

BUG=none
TEST=manual tests

Change-Id: Ie4346ef6df1282ec3e3f8045a08138c93653fece
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/320735
Tested-by: Jack Neus <jackneus@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/init.py

@@ -127,6 +127,11 @@ to update the working directory files.
     # anew.
     if not is_new:
       was_standalone_manifest = m.config.GetString('manifest.standalone')
+      if was_standalone_manifest and not opt.manifest_url:
+        print('fatal: repo was initialized with a standlone manifest, '
+              'cannot be re-initialized without --manifest-url/-u')
+        sys.exit(1)
+
       if opt.standalone_manifest or (
           was_standalone_manifest and opt.manifest_url):
         m.config.ClearCache()