ra: Persist link flag in FileState when replaying revisions
When adding a svn link that will be exported as a symbolic link, that information must be persisted across revisions replay.
Previously if a link was modified in another revision, like for instance adding svn:eol-style property, this could lead to removing the exported symbolic link while it should not.
Related to #3695
Migrated from D6702 (view on Phabricator)