sync: Fix locking problems during merge when annex.pidlock is set

sync: Fix locking problems during merge when annex.pidlock is set

Presumably git merge sometimes needs to verifiy if a worktree file is
modified, and so will then run git-annex filter-process which would try to
take the pid lock. And for whatever reason, git-annex sync already had the
pidlock held. I have not replicated that, but it does make enough sense to
deploy the workaround.

Like I said back in commit 7bdb0cdc0d6f35d3835e57c3a34740cc98f89719,

   Arguably, it would be better to have a way to make any process git-annex
   runs have the env var set. But then it would need to take the pid lock
   when running any and all processes, and that would be a problem when
   git-annex runs two processes concurrently. So, I'm left doing it ad-hoc
   in places where git-annex really does run a child process, directly
   or indirectly via a particular git command.

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