Avoid repeated checking that files passed on the command line exist.

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.

Command/Add.hs

@@ -63,7 +63,8 @@ seek o = allowConcurrentOutput $ do
 				giveup "--update --batch is not supported"
 			| otherwise -> batchFiles gofile
 		NoBatch -> do
-			let go a = a gofile (addThese o)
+			l <- workTreeItems (addThese o)
+			let go a = a gofile l
 			unless (updateOnly o) $
 				go (withFilesNotInGit (not $ includeDotFiles o))
 			go withFilesMaybeModified