pre tarred the entire HERMES_HOME, which is slow on a real home (hundreds of
thousands of files) and silently left out files other processes held open:
on a live 33GB home the tar came out at 4.7GB with no error.
pre now takes a `hermes backup` of the user's data, then a Volume Shadow
Copy snapshot of the volume(s) holding HERMES_HOME and the Electron userData.
The snapshot is copy-on-write: 2s, nothing copied, locked files included.
post checks every snapshot still exists before touching anything, then
robocopy /MIR's both trees back from it (only changed files are copied,
files the update added are deleted; HERMES_HOME\cache is left alone), and
deletes the snapshot. If Windows dropped the snapshot, post changes nothing
and points at the hermes backup zip instead.
pre and post now need an elevated PowerShell.
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