drop, move, mirror: when two files have the same content, honor the max numcopies...

drop, move, mirror: when two files have the same content, honor the max numcopies and requiredcopies

Eg, before with a .gitattributes like:

*.2 annex.numcopies=2
*.1 annex.numcopies=1

And foo.1 and foo.2 having the same content and key, git-annex drop foo.1 foo.2
would succeed, leaving just 1 copy, despite foo.2 needing 2 copies.
It dropped foo.1 first and then skipped foo.2 since its content was gone.

Now that the keys database includes locked files, this longstanding wart
can be fixed.

Sponsored-by: Noam Kremen on Patreon