t7400: clarify 'submodule add' tests
A new reader may not realize what properties the $submodurl
repository needs to have.
One of the tests is checking that ‘submodule add -b foo’ creates
a ‘foo’ branch. Put this test in context by checking that
without -b, no ‘foo’ branch is created.
While at it, make sure each added submodule is a reasonable
repository, with clean index, no stray files, and so on.
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>