dir: refactor treat_directory to clarify control flow

The logic in treat_directory() is handled by a multi-case switch statement, but this switch is very asymmetrical, as the first two cases are simple but the third is more complicated than the rest of the method. In fact, the third case includes a "break" statement that leads to the block of code outside the switch statement. That is the only way to reach that block, as the switch handles all possible values from directory_exists_in_index(); Extract the switch statement into a series of "if" statements. This simplifies the trivial cases, while clarifying how to reach the "show_other_directories" case. This is particularly important as the "show_other_directories" case will expand in a later change. Helped-by: Elijah Newren <newren@gmail.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>