git annex add, git annex lock etc make multiple seek passes,
and each seek pass checked that files existed. That was unncessary
redundant work.
Fixed by adding a new WorkTreeItem type, make seek actions use it,
and check that the files exist when constructing it.
This commit was supported by the NSF-funded DataLad project.
Before, there was a window where interrupting an add could result in the
file being moved into the annex, with no symlink yet created.
This commit was supported by the NSF-funded DataLad project.