### Please describe the problem.
If `git annex fsck --from web` encounters a URL that is no longer available,
it will remove all URLs from the appropriate file's location log.
Surprisingly, adding one of the URLs the file was associated to before brings back all of them.

### What steps will reproduce the problem?

    $ git annex whereis file.txt
    whereis file.txt (3 copies) 
  	    ...

      The following untrusted locations may also have copies:
  	    00000000-0000-0000-0000-000000000001 -- web

      web: http://example.org/dead/file.txt
      web: http://example.org/alive/file.txt
    ok
    $ git annex fsck --from web
    fsck file.txt (checking http://example.org/dead/file.txt...) (fixing location log)
      ** Based on the location log, file.txt
      ** was expected to be present, but its content is missing.
    failed
    (recording state in git...)
    git-annex: fsck: 1 failed
    $ git annex whereis file.txt
    whereis file.txt (3 copies) 
  	    ...
    ok
    $ git annex addurl http://example.org/alive/file.txt
    addurl file.txt ok
    (recording state in git...)
    $ git annex whereis file.txt
    whereis file.txt (3 copies) 
  	    ...

      The following untrusted locations may also have copies:
  	    00000000-0000-0000-0000-000000000001 -- web

      web: http://example.org/dead/file.txt
      web: http://example.org/alive/file.txt
    ok
    

The only fix that seems to fix this is using `rmurl` on the dead one.

### What version of git-annex are you using? On what operating system?
    $ git annex version
    git-annex version: 6.20171026
    build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify ConcurrentOutput TorrentParser MagicMime Feeds Quvi
    dependency versions: aws-0.16 bloomfilter-2.0.1.0 cryptonite-0.23 DAV-1.3.1 feed-0.3.12.0 ghc-8.0.2 http-client-0.5.7.0 persistent-sqlite-2.6.3 torrent-10000.1.1 uuid-1.3.13 yesod-1.4.5
    key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 SHA1E SHA1 MD5E MD5 WORM URL
    remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository versions: 3 5 6
    upgrade supported from repository versions: 0 1 2 3 4 5
    operating system: linux x86_64

I'm running NixOS btw.

### 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)
I'm lovin' it!

> [[fixed|done]] --[[Joey]] 
