This fixes a regression created by fe1230dece81450004d02fa8a470f8dab8f7fdd9,
and modifies a test to avoid future regressions.
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Both cgit_print_diff() and cgit_diff_tree() handles root commits nicely,
but cgit_print_commit() forgot to check the case of 0 parents.
This fixes it, and adds tests to avoid future regressions.
Signed-off-by: Lars Hjemli <hjemli@gmail.com>