Clean up some uses of showStart with "" for the file,
or in some cases, a non-filename description string. That would
generate bad json, although none of the commands doing that
supported --json.
Using "" for the file resulted in output like "foo rest";
now the extra space is eliminated.
This commit was sponsored by Fernando Jimenez on Patreon.
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.
