metadata --remove stalls as of 88a7fb5cb

With 88a7fb5cb (convert all applicable commands to new 2x faster annexed file seeking, 2020-07-10), the following script hangs on the last command.

cd "$(mktemp -d ${TMPDIR:-/tmp}/ga-metadata-XXXXXXX)"
git init
git annex init
echo 0 >f0
echo 1 >f1
git annex add f0 f1
git commit -mc0
git annex metadata --debug --remove a .

Here's the output, where the git-ls-files call is the spot of the hang:

[... 13 lines ...]
[2020-07-10 21:04:33.030754298] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","symbolic-ref","-q","HEAD"]
[2020-07-10 21:04:33.032022677] process done ExitSuccess
[2020-07-10 21:04:33.032205649] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","refs/heads/master"]
[2020-07-10 21:04:33.033970479] process done ExitSuccess
[2020-07-10 21:04:33.034308573] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","cat-file","--batch=%(objectname) %(objecttype) %(objectsize)","--buffer"]
[2020-07-10 21:04:33.035040652] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","git-annex"]
[2020-07-10 21:04:33.037070231] process done ExitSuccess
[2020-07-10 21:04:33.037281793] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","--hash","refs/heads/git-annex"]
[2020-07-10 21:04:33.038995286] process done ExitSuccess
[2020-07-10 21:04:33.03938464] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","log","refs/heads/git-annex..d39f6bdff82a20be7edf466ba572877de0db0daf","--pretty=%H","-n1"]
[2020-07-10 21:04:33.041697801] process done ExitSuccess
[2020-07-10 21:04:33.042060234] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","cat-file","--batch"]
[2020-07-10 21:04:33.047868787] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","cat-file","--batch-check=%(objectname) %(objecttype) %(objectsize)"]
[2020-07-10 21:04:33.049298291] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","ls-files","--stage","-z","--","."]
[2020-07-10 21:04:42.432722123] process done ExitFailure (-2)

On the parent of the commit, this completes fine, with the expected "use --force" failure.

[... 13 lines ...]
[2020-07-10 21:02:36.753459763] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","symbolic-ref","-q","HEAD"]
[2020-07-10 21:02:36.756144724] process done ExitSuccess
[2020-07-10 21:02:36.756300697] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","show-ref","refs/heads/master"]
[2020-07-10 21:02:36.758083727] process done ExitSuccess
[2020-07-10 21:02:36.758319366] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","ls-files","-z","--cached","--","."]
git-annex: Not recursively setting metadata. Use --force to do that.