fail if git only repository, or if there is no readme

error messages should be corrected, it is not always clear what goes wrong.

Submodules cannot be .noannex repository.
If there is no readme file in one submodule, the code fails because there is nothing to commit.

Start by reproducing the failure with a repository containing only Git metadata and with a submodule that has no README. Trace the resulting error messages and repository handling, then verify that each case reports what went wrong instead of failing because there is nothing to commit.

Written by the indexing model from the issue text.