dir: add generic "walk all files" helper

dir: add generic "walk all files" helper

There is sometimes a need to visit every file within a directory,
recursively. The main example is remove_dir_recursively(), though it has
some extra flags that make it want to iterate over paths in a custom
way. There is also the fill_directory() approach but that involves an
index and a pathspec.

This change adds a new for_each_file_in_dir() method that will be
helpful in the next change.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>