Do not expect unlink(2) to fail on a directory.

Do not expect unlink(2) to fail on a directory.

When "git checkout-index" checks out path A/B/C, it makes sure A
and A/B are truly directories; if there is a regular file or
symlink at A, we prefer to remove it.

We used to do this by catching an error return from mkdir(2),
and on EEXIST did unlink(2), and when it succeeded, tried
another mkdir(2).

Thomas Glanzmann found out the above does not work on Solaris
for a root user, as unlink(2) was so old fashioned there that it
allowed to unlink a directory.

As pointed out, this still doesn't guarantee that git won't call
"unlink()" on a directory (race conditions etc), but that's
fundamentally true (there is no "funlink()" like there is
"fstat()"), and besides, that is in no way git-specific (ie it's
true of any application that gets run as root).

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>