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/Forget.hs

@@ -34,7 +34,7 @@ seek = commandAction . start
 
 start :: ForgetOptions -> CommandStart
 start o = do
-	showStart "forget" "git-annex"
+	showStart' "forget" (Just "git-annex")
 	c <- liftIO currentVectorClock
 	let basets = addTransition c ForgetGitHistory noTransitions
 	let ts = if dropDead o