Fix display problem of clocked time in agenda

Fix display problem of clocked time in agenda

* lisp/org-agenda.el (org-agenda-get-progress): Avoid reusing previous
value of EXTRA.

Sebastien Vauban writes:

> Finally reporting the following bug. Has been there for as long as I can
> remember, but never did report it when seeing it. And kept forgetting.
>
> So, now...
>
> --8<---------------cut here---------------start------------->8---
> * 2010
>
> ** 2010-06 June
>
> *** Admin
>
> **** Organization
>     :LOGBOOK:
>     CLOCK: [2010-06-30 Wed 13:30]--[2010-06-30 Wed 17:50] =>  4:20
>     - DUPLICATED TeXt.
>     :END:
>
> **** Emails and News
>     :LOGBOOK:
>     CLOCK: [2010-06-28 Mon 09:10]--[2010-06-28 Mon 10:40] =>  1:30
>     CLOCK: [2010-06-30 Wed 10:30]--[2010-06-30 Wed 12:30] =>  2:00
>     :END:
>
> *** ABC
>    :LOGBOOK:
>    CLOCK: [2010-06-30 Wed 09:30]--[2010-06-30 Wed 10:30] =>  1:00
>    - Transfer of files to ABC.
>    :END:
> --8<---------------cut here---------------end--------------->8---
>
> generates the following timeline:
>
> --8<---------------cut here---------------start------------->8---
> Timeline of file /home/sva/Projects/ecm.org
>
> 2010-06-28 Mon _________________________________
>  Clocked:   (1:30) Emails and News
>
> 2010-06-29 Tue _________________________________
>
> 2010-06-30 Wed _________________________________
>  Clocked:   (1:00) ABC - Transfer of files to ABC.
>  Clocked:   (2:00) Emails and News - DUPLICATED TeXt.
>  Clocked:   (4:20) Organization - DUPLICATED TeXt.
> --8<---------------cut here---------------end--------------->8---
>
> As you can see, the fact that I never put a descriptive text for "reading
> emails" is wrongly reported: previous text is used in the timeline, instead.