Re: [PATCH v2 1/1] git clone: don't clone into non-empty directory

Ben Wijen [off-list ref] writes:

When using git clone with --separate-git-dir realgitdir and
realgitdir already exists, it's content is destroyed.

So, make sure we don't clone into an existing non-empty directory.

This came from d45420c1 (clone: do not clean up directories we
didn't create, 2018-01-02), where we claimed

    Because we know that the only directory we'll write into is
    an empty one, we can handle this case by just passing the
    KEEP_TOPLEVEL flag to our recursive delete (if we could
    write into populated directories, we'd have to keep track of
    what we wrote and what we did not, which would be much
    harder).

The assumed use case is that somebody created an empty directory for
our use in a grandparent directory where we have no write permission
and gave it to us, and we want to do a "git clone" into it while
keeping that directory alive.  But we didn't make sure the directory
was not empty ourselves---we just assumed it to be empty.

And the originally envisioned use case does not have anything to do
with the use of --separate-git-dir at all.  So from that point of
view, this change does not break the originally envisioned use case,
so this probably is not regressing any existing and valid use case
that may not involve "--separate-git-dir", but I'd rather see that
the commit message explain these things to its readers to justify
itself.

Thanks.