main: fix --mkpath + --dry-run file-to-file copy (#880)

main: fix --mkpath + --dry-run file-to-file copy (#880)

A single-file --mkpath copy whose destination parent does not exist
failed under --dry-run: make_path() only *reports* the directories it
would create in a dry run, so change_dir#3 then tried to chdir into a
parent that isn't there and aborted with "change_dir#3 ... failed".

When the parent is genuinely missing in a dry run, skip the chdir and
mark the destination as not-yet-present (dry_run++), exactly as the
multi-file/dir-creation path already does, so the generator doesn't
probe the missing tree.  Gating it on the missing-parent case keeps an
ordinary file-to-file dry run chdir'ing into and itemizing against an
existing destination.

Fixes: #880
Co-authored-by: Stiliyan Tonev (Bark) <stiliyan21@gmail.com>