For example, this could happen if using SHA1 and a file with content
"foo" were added to that backend. Then a file with "content" foo were
migrated from the WORM backend.
Assume that, if a backend assigned the same key, the already annexed
content must be the same. So, the "old" content can be reused.
Free space checking is now done, for transfers of data for keys that have free space metadata.
(Notably, not for SHA* keys generated with git-annex 0.24 or earlier.)
The code is believed to work on Linux, FreeBSD, and OSX; check compile-time
messages to see if it is not enabled for your OS.
Rename Locations functions for better consitency, and make their values
more consistent too.
Used </> rather than manually building paths. There are still more places
that manually do so, but are tricky, due to the behavior of </> when
the second FilePath is absolute. So I only changed places where
it obviously was relative.
