diff: add tests for --relative without optional prefix value
We already have tests for --relative, but they currently only test when
a prefix has been provided. This fails to test the case where --relative
by itself should use the current directory as the prefix.
Teach the check_$type functions to take a directory argument to indicate
which subdirectory to run the git commands in. Add a new test which uses
this to test --relative without a prefix value.
Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>