bug#29205: --force doesn't work

[Konstantin Kharlamov]

Steps to reproduce:

1. $ mkdir -p foo/bar/buzz1
2. $ mkdir -p bar/buzz2
3. $ mv --force bar foo/

Expected result: "bar" is merged into the other "bar"
Actual result: error "mv: cannot move 'bar' to 'foo/bar': Directory not 
empty"

I am experiencing this the second time on this week, it's a real 
problem. I'm building a package from a source code, but then it fails 
because it's another buggy app, so I'm doing "mv 
/tmp/yaourt-blahblha/package-name ~/Projects" set out to see if I can 
fix the code, and send patches.

But then "mv" fails, because Archlinux's `makepkg` is in its turn buggy, 
and leaves bad permissions on `pkg` dir inside the code.

So I'm removing this dir, and trying to continue `mv`ing, and here we 
coming to the bug I'm reporting.

Of course I could just copy, but `mv`ing is α) much faster, and β) 
leaves dates of file creation in places, so I don't need to rebuild the 
whole thing over again, only the files I gonna change.