showStart variant for when there's no worktree file

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.

Command/Dead.hs

@@ -33,7 +33,7 @@ seek (DeadKeys ks) = seekActions $ pure $ map startKey ks
 
 startKey :: Key -> CommandStart
 startKey key = do
-	showStart "dead" (key2file key)
+	showStart' "dead" (Just $ key2file key)
 	ls <- keyLocations key
 	case ls of
 		[] -> next $ performKey key