global numcopies setting

* numcopies: New command, sets global numcopies value that is seen by all
  clones of a repository.
* The annex.numcopies git config setting is deprecated. Once the numcopies
  command is used to set the global number of copies, any annex.numcopies
  git configs will be ignored.
* assistant: Make the prefs page set the global numcopies.

This global numcopies setting is needed to let preferred content
expressions operate on numcopies.

It's also convenient, because typically if you want git-annex to preserve N
copies of files in a repo, you want it to do that no matter which repo it's
running in. Making it global avoids needing to warn the user about gotchas
involving inconsistent annex.numcopies settings.
(See changes to doc/numcopies.mdwn.)

Added a new variety of git-annex branch log file, that holds only 1 value.
Will probably be useful for other stuff later.

This commit was sponsored by Nicolas Pouillard.

Command/Drop.hs

@@ -139,7 +139,7 @@ notEnoughCopies key need have skip bad = do
 	return False
   where
 	unsafe = showNote "unsafe"
-	hint = showLongNote "(Use --force to override this check, or adjust annex.numcopies.)"
+	hint = showLongNote "(Use --force to override this check, or adjust numcopies.)"
 
 {- In auto mode, only runs the action if there are enough
  - copies on other semitrusted repositories.