t9300 (fast-import): another test for the "replace root" feature

t9300 (fast-import): another test for the "replace root" feature

Another test for the replace root feature.  One can imagine an
implementation for which R "some/subdir" "" would free some state
associated to the subdir and leave fast-import confused.

Luckily, git's is not such an implementation.

While at it, change the previous test to use C "some/subdir" ""
instead of R (i.e., test both syntaxes).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>