Importing from a special remote honors its preferred content too; unwanted
files are not imported. But, some preferred content expressions can't be
checked before files are imported, and trying to import with such an
expression will fail.
Tested this with scenarios including changing the preferred content
expression and making sure merging the import didn't delete files that were
no longer wanted.
There was one minor inefficiency mentioned in the todo that I punted on.
This includes a note about how include= and exclude= match when exporting
a subtree. I don't know if the note is prominent enough, but the
behavior seems unsurprising enough.
When a repo is in multiple standard groups,
Logs.PreferredContent.makeMatcher uses unknownMatcher,
so will not get everything, but instead defaults to wanting whatever is
present.
Added --securehash option to match files using a secure hash function, and
corresponding securehash preferred content expression.
This commit was sponsored by Ethan Aubin.
