New test: "stg diff"

A simple test to make sure that we can run "stg diff" without
arguments, just to list local changes.

Note that two subtests currently fail; these are due to plain "stg
diff" failing on a branch where "stg init" hasn't been run, which is
plainly a bug.

Signed-off-by: Karl Hasselström <kha@treskal.com>