feat: upgrade existing filesystem

[upils]

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This commit enables Chisel detecting the target directory contains the result
of a previous execution to then operate an upgrade of the content. This initial
simple implementation has the the limitation that content (files, symlinks) is
systematically replaced, even if identical. As a consequences:

• user modifications on chisel-managed content is overridden.
• in the context of a OCI image build, this "new" content is identified as
  different and thus the new OCI layer contains duplicated files.

[github-actions]

Command	Mean [s]	Min [s]	Max [s]	Relative
BASE	12.985 ± 0.470	12.425	13.668	1.02 ± 0.05
HEAD	12.784 ± 0.374	12.191	13.252	1.00