diagnose.c: refactor to safely use 'd_type'

diagnose.c: refactor to safely use 'd_type'

Refactor usage of the 'd_type' property of 'struct dirent' in 'diagnose.c'
to instead utilize the compatibility macro 'DTYPE()'. On systems where
'd_type' is not present in 'struct dirent', this macro will always return
'DT_UNKNOWN'. In that case, instead fall back on using the 'stat.st_mode' to
determine whether the dirent points to a dir, file, or link.

Additionally, add a test to 't0092-diagnose.sh' to verify that files (e.g.,
loose objects) are counted properly.

Note that the new function 'get_dtype()' is based on 'resolve_dtype()' in
'dir.c' (which itself was refactored from a prior 'get_dtype()' in
ad6f2157f9 (dir: restructure in a way to avoid passing around a struct
dirent, 2020-01-16)), but differs in that it is meant for use on arbitrary
files, such as those inside the '.git' dir. Because of this, it does not
search the index for a matching entry to derive the 'd_type'.

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>