Three_tests_fail_when_annex.backends_is_defined.mdwn

### Please describe the problem.

I noticed three tests failed when running "git annex test", and it seems 
it's because I have `annex.backends` set to `SHA256` in `~/.gitconfig`. 
When I disable that option, all tests succeed.

### What steps will reproduce the problem?

Add

    [annex]
      backends = SHA256

to `~/.gitconfig`.

### What version of git-annex are you using? On what operating system?

Newest git-annex (6.20160211 amd64) from downloads.kitenet.net.

- Debian GNU/Linux 8.3 (64 bit). Installed yesterday, so it's pretty pristine.
- git version 2.7.1.287.g4943984 (Newest version from 'master' in 
  git.git)

The first version with this problem is 6.20160114. 5.20151218 works.

### Please provide any additional information below.

[[!format text """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

$ git annex test
Tests

[Removed 188 lines]

    migrate (via gitattributes):                          /etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
OK (3.11s)
    unused:                                               /etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
FAIL (1.84s)
      unused keys differ after origin branches are gone
      expected: [Key {keyName = "e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77", keyBackendName = "SHA256E", keySize = Just 20, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}]
       but got: [Key {keyName = "e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77", keyBackendName = "SHA256", keySize = Just 20, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}]
    describe:                                             /etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
OK (0.84s)

[Removed 1108 lines]

    migrate (via gitattributes):                          /etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
OK (3.34s)
    unused:                                               /etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
/etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
/etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
FAIL (1.82s)
      unused keys differ after origin branches are gone
      expected: [Key {keyName = "e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77", keyBackendName = "SHA256E", keySize = Just 20, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}]
       but got: [Key {keyName = "e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77", keyBackendName = "SHA256", keySize = Just 20, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}]
    describe:                                             /etc/magic, 4: Warning: using regular magic file `/usr/share/misc/magic'
OK (0.78s)

[Removed 1062 lines]

    migrate (via gitattributes):                          OK (2.64s)
    unused:                                               FAIL (1.53s)
      unused keys differ after origin branches are gone
      expected: [Key {keyName = "e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77", keyBackendName = "SHA256E", keySize = Just 20, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}]
       but got: [Key {keyName = "e394a389d787383843decc5d3d99b6d184ffa5fddeec23b911f9ee7fc8b9ea77", keyBackendName = "SHA256", keySize = Just 20, keyMtime = Nothing, keyChunkSize = Nothing, keyChunkNum = Nothing}]
    describe:                                             OK (0.62s)

[Removed 1667 lines]

3 out of 269 tests failed (640.58s)
  (This could be due to a bug in git-annex, or an incompatability
   with utilities, such as git, installed on this system.)
$

# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Roses are red<br/>
Violets are blue<br/>
git-annex is awesome<br/>
and so are you

`;-)`

But bloody hell, it's hard to get this thing to build. As you've 
mentioned earlier, building on Debian 7 isn't supported anymore, so I 
installed Debian 8.3 yesterday for the sole purpose of compiling git-annex, 
but still no luck:

    [32 of 32] Compiling Main             ( dist/setup/setup.hs, dist/setup/Main.o )
    Linking ./dist/setup/setup ...
    unrecognized option `--extra-prog-path=/home/sunny/.cabal/bin'
    Makefile:19: recipe for target 'Build/SysConfig.hs' failed
    make: *** [Build/SysConfig.hs] Error 1

I was hoping to get it to build so I could find the exact commit that 
introduced this, and also provide some patches with some functionality 
I've been thinking of. Is building on Debian 8.3 (jessie) supported? I 
found a bug report from you on 
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=778987> that seems to 
be related, but I don't know if that's the case. The newest version I'm 
able to build is 5.20150219, somewhere after that various things start 
to fail.