dir.[ch]: add 'follow_symlink' arg to 'get_dtype'

dir.[ch]: add 'follow_symlink' arg to 'get_dtype'

Add a 'follow_symlink' boolean option to 'get_type()'. If 'follow_symlink'
is enabled, DT_LNK (in addition to DT_UNKNOWN) d_types triggers the
stat-based d_type resolution, using 'stat' instead of 'lstat' to get the
type of the followed symlink. Note that symlinks are not followed
recursively, so a symlink pointing to another symlink will still resolve to
DT_LNK.

Update callers in 'diagnose.c' to specify 'follow_symlink = 0' to preserve
current behavior.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>