Prevents merging the import from deleting the non-preferred files from
the branch it's merged into.
adjustTree previously appended the new list of items to the old, which
could result in it generating a tree with multiple files with the same
name. That is not good and confuses some parts of git. Gave it a
function to resolve such conflicts.
That allowed dealing with the problem of what happens when the import
contains some files (or subtrees) with the same name as files that were
filtered out of the export. The files from the import win.
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.
The filtering is fairly efficient as far as building the trees goes,
since it reuses adjustTree. But it still needs to traverse the whole
tree, and look up the keys used by every file.
The tree that gets recorded to export.log is the filtered tree.
This way resumes of interrupted sync to an export uses it without
needing to recalculate it. And, a change to the preferred content
settings of the remote will result in a different tree, so the export
will be updated accordingly.
The original tree is still used in the remote tracking branch.
That branch represents the special remote as a git remote, and if it
were a normal git remote, the tree in its head would not be affected by
preferred content.
Only when the preferred content expression includes them will a parse
failure due to them needing keys result in the preferred content
expression not parsing in keyless mode.
This will let import try to match preferred content expressions before
downloading the content and generating its key.
If an expression needs a key, it preferredContentParser with
preferredContentKeylessTokens will fail to parse it.
standard and groupwanted are not in preferredContentKeylessTokens
because they may refer to an expression that refers to a key.
That needs further work to support them.
