rebase: improve error message for empty --source set Before, it just said 'nothing to rebase' in this case. Now, it aborts mentioning the reason: 'empty "source" revision set'. Specifying revisions that cannot be rebased is a 'soft' error, but specifying an empty set deserves an abort that explains exactly what the problem is.

Hello, Foobar
['Hello, Fo, =;o', 'Hello, Bar']