### Please describe the problem.

If there are multiple files with the same keys in the repository and they are copied to bup special remote,
then `git annex fsck --from=bup` with `--jobs=N` option (N >= 2) can show an error and remove these keys from bup.

Based on the error message (about locked .git/annex/tmp/ file), this problem is probably not specific to bup,
but I tested it with bup only.

### What steps will reproduce the problem?

1. Configure a bup special remote.
2. Add files with the same content to annex (and with the same backend).
3. Copy these files to bup.
4. Run `git annex fsck --from=bup -JN` several times, until it removes these keys from bup.

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

git-annex 7.20191230-g985373f8e, build from source, on Debian GNU/Linux buster.

bup 0.29.3-2 from Debian sid. Also tried with bup 0.30, build from source.

### Please provide any additional information below.

[[!format txt """
~ $ mkdir testdir
~ $ cd testdir
~/testdir $ 
~/testdir $ git init
Initialized empty Git repository in /home/test/testdir/.git/
~/testdir $ 
~/testdir $ git annex init testrepo
init testrepo (scanning for unlocked files...)
ok
(recording state in git...)
~/testdir $ 
~/testdir $ ls ~/.bup/index-cache/
~/testdir $ 
~/testdir $ git annex initremote bup type=bup buprepo=~/testdir/.bup encryption=none
initremote bup (bup init...) 
Reinitialized existing Git repository in /home/test/.bup/
Initialized empty Git repository in /home/test/testdir/.bup/
ok
(recording state in git...)
~/testdir $ 
~/testdir $ ls ~/.bup/index-cache/
None__home_test_testdir__bup
~/testdir $ 
~/testdir $ echo aaa >file1
~/testdir $ echo aaa >file2
~/testdir $ 
~/testdir $ git annex add .
add file1 
ok                                
add file2 
ok                                
(recording state in git...)
~/testdir $ 
~/testdir $ git commit -m files
[master (root-commit) 7a03b66] files
 2 files changed, 2 insertions(+)
 create mode 120000 file1
 create mode 120000 file2
~/testdir $ 
~/testdir $ git -C .bup show-ref
~/testdir $ 
~/testdir $ git annex whereis
whereis file1 (1 copy) 
  	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
ok
whereis file2 (1 copy) 
  	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
ok
~/testdir $ 
~/testdir $ git annex copy --to=bup .
copy file1 (to bup...) 

bloom: creating from 1 file (3 objects).ing: 0 kbytes
Receiving index from server: 1156/1156, done.
bloom: creating from 1 file (3 objects).
ok                                
copy file2 ok
(recording state in git...)
~/testdir $ 
~/testdir $ git annex lookupkey file1 file2
SHA256E-s4--17e682f060b5f8e47ea04c5c4855908b0a5ad612022260fe50e11ecb0cc0ab76
SHA256E-s4--17e682f060b5f8e47ea04c5c4855908b0a5ad612022260fe50e11ecb0cc0ab76
~/testdir $ 
~/testdir $ git -C .bup show-ref
2076647ee23ad632c8cf96caf51febbd0604452c refs/heads/SHA256E-s4--17e682f060b5f8e47ea04c5c4855908b0a5ad612022260fe50e11ecb0cc0ab76
~/testdir $ 
~/testdir $ git annex fsck --from=bup
fsck file1 
(checksum...) ok                                    
fsck file2 
(checksum...) ok                                    
(recording state in git...)
~/testdir $ 
~/testdir $ git -C .bup show-ref
2076647ee23ad632c8cf96caf51febbd0604452c refs/heads/SHA256E-s4--17e682f060b5f8e47ea04c5c4855908b0a5ad612022260fe50e11ecb0cc0ab76
"""]]

Now run `git annex fsck --from=bup -J2` multiple times, until it drops the key from bup...

[[!format txt """
~/testdir $ git annex fsck --from=bup -J2
fsck file1 fsck file2 

100%  4 B                 5 B/s 0s                  
  content cannot be completely removed from bup remote

  file2: Bad file size (4 B smaller); dropped from bup
(checksum...) 
git-annex: .git/annex/tmp/fsck14654.SHA256E-s4--17e682f060b5f8e47ea04c5c4855908b0a5ad612022260fe50e11ecb0cc0ab76: openBinaryFile: resource busy (file is locked)
failed
(fixing location log) (checksum...) ok
(recording state in git...)
git-annex: fsck: 1 failed
~/testdir $ 
~/testdir $ git -C .bup show-ref
~/testdir $ 
~/testdir $ git annex whereis
whereis file1 (2 copies) 
  	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
   	88cc362a-f87a-43c7-b194-e79b2ee91828 -- [bup]
ok
whereis file2 (2 copies) 
  	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
   	88cc362a-f87a-43c7-b194-e79b2ee91828 -- [bup]
ok
~/testdir $ 
~/testdir $ git annex fsck --from=bup 
fsck file1 (fixing location log) 
  ** Based on the location log, file1
  ** was expected to be present, but its content is missing.
failed
fsck file2 ok
(recording state in git...)
git-annex: fsck: 1 failed
~/testdir $ 
~/testdir $ git annex whereis
whereis file1 (1 copy) 
  	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
ok
whereis file2 (1 copy) 
  	5d9b0df2-000b-4273-bc4a-fb3b9d8319bd -- testrepo [here]
ok
"""]]

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