hpd-git-resync: new command to resume Git mirrorring

Useful if Git mirrorring was suspended for experimentation
of fully native Mercurial without Git, but resuming is needed.

Part of heptapod#580

(perhaps call it `hpd-resume-git-mirrorring`?)
5 jobs for !74 with topic/default/git-resync in 5 minutes and 11 seconds (queued for 3 seconds)
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