Before this change, bisync supported `--backup-dir` only when `Path1` and
`Path2` were different paths on the same remote. With this change, bisync
introduces new `--backup-dir1` and `--backup-dir2` flags to support separate
backup-dirs for `Path1` and `Path2`.
`--backup-dir1` and `--backup-dir2` can use different remotes from each other,
but `--backup-dir1` must use the same remote as `Path1`, and `--backup-dir2`
must use the same remote as `Path2`. Each backup directory must not overlap its
respective bisync Path without being excluded by a filter rule.
The standard `--backup-dir` will also work, if both paths use the same remote
(but note that deleted files from both paths would be mixed together in the
same dir). If either `--backup-dir1` and `--backup-dir2` are set, they will
override `--backup-dir`.
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