Previously, I used a shell script to extract modification date ranges
from the Git history, but that was complicated and unreliable, so now
every file just gets the same year range in its copyright header. If
someone needs to know when a specific file was modified and by whom,
they can look at the Git history themselves; no need to include it
redundantly in the header.
The whole point of PersistentSet is to provide non-destructive write
operations, which means the add and remove methods should have no
effect on previous revisions. However, a bug in remove caused shared
tree nodes to be modified, corrupting any revisions with which they
were shared.
In PersistentSet.remove, we were modifying the child node in place
instead of making a copy to update, which would corrupt older
revisions. This commit ensures that we always create a copy if
necessary.
