add gitAnnexChildProcess and use instead of incorrect use of runsGitAnnexChildProcess

add gitAnnexChildProcess and use instead of incorrect use of runsGitAnnexChildProcess

Fixes reversion in 8.20200617 that made annex.pidlock being enabled result
in some commands stalling, particularly those needing to autoinit.

Renamed runsGitAnnexChildProcess to make clearer where it should be
used.

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.