Previously, Custody::absolute_path() was called for every call to
validate_path_against_process_veil(). For processes that don't have a
veil, the path is not used by the function. This means that it is
unnecessarily generated. This introduces an overload to
validate_path_against_process_veil(), which takes a Custody const& and
only generates the absolute path if it there is actually a veil and it
is thus needed.
This patch results in a speed up of Assistant's file system cache
building by around 16 percent.
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