When a file is annexed, a key is generated from its content and/or metadata.
The file checked into git symlinks to the key. This key can later be used
to retrieve the file's content (its value).

Multiple pluggable key-value backends are supported, and a single repository
can use different ones for different files.

* `SHA256E` -- The default backend for new files, combines a 256 bit SHA-2
  hash of the file's content with the file's extension. This allows
  verifying that the file content is right, and can avoid duplicates of
  files with the same content. Its need to generate checksums
  can make it slower for large files. 
* `SHA256` -- SHA-2 hash that does not include the file extension in the
  key, which can lead to better deduplication but can confuse some programs.
* `SHA512`, `SHA512E` -- Best SHA-2 hash, for the very paranoid.
* `SHA384`, `SHA384E`, `SHA224`, `SHA224E` -- SHA-2 hashes for
  people who like unusual sizes.
* `SHA3_512`, `SHA3_512E`, `SHA3_384`, `SHA3_384E`, `SHA3_256`, `SHA3_256E`, `SHA3_224`, `SHA3_224E`
  -- SHA-3 hashes, for bleeding edge fun.
* `SKEIN512`, `SKEIN512E`, `SKEIN256`, `SKEIN256E`
  -- [Skein hash](http://en.wikipedia.org/wiki/Skein_hash),
  a well-regarded SHA3 hash competition finalist.
* `SHA1`, `SHA1E`, `MD5`, `MD5E` -- Smaller hashes than `SHA256`
   for those who want a checksum but are not concerned about security.
* `WORM` ("Write Once, Read Many") -- This assumes that any file with
  the same filename, size, and modification time has the same content.
  This is the least expensive backend, recommended for really large
  files or slow systems.
* `URL` -- This is a key that is generated from the url to a file.
  It's generated when using eg, `git annex addurl --fast`, when the file
  content is not available for hashing.

Note that the various 512 and 384 length hashes result in long paths,
which are known to not work on Windows. If interoperability on Windows is a
concern, avoid those.

The `annex.backends` git-config setting can be used to list the backends
git-annex should use. The first one listed will be used by default when
new files are added.

For finer control of what backend is used when adding different types of
files, the `.gitattributes` file can be used. The `annex.backend`
attribute can be set to the name of the backend to use for matching files.

For example, to use the SHA256E backend for sound files, which tend to be
smallish and might be modified or copied over time,
while using the WORM backend for everything else, you could set
in `.gitattributes`:

	* annex.backend=WORM
	*.mp3 annex.backend=SHA256E
	*.ogg annex.backend=SHA256E
