1. Summarise the issue (what were doing, what went wrong?)

Running `optimize --reorder` seems to never end on this repository (let
it run for 2 hours and it did not finish).

Here is the problematic repository:

https://dl.dropboxusercontent.com/u/6239815/FORENSIC.opt.reorder.120min.reviewed.tar.gz
 (< 48 MBytes)


2. What behaviour were you expecting instead?

Taking a reasonable amount of time to run, and also report progress
(currently `optimize --reorder` is silent save for the last output message).

3. Version used

Darcs 2.9.8 

4. What operating system are you running?

Ubuntu Linux 14.04

Ale and I saw that the call to 'removeFromUnrevertContext' from the
function 'tentativelyReplacePatches' called by 'optimize --reorder' was
the source of the problem.

We are going to fix the issue by not updating anymore the unrevert patch
when reordering history (since reordering does not remove any patch in
the end, the unrevert patch will always be appliable).

However it will be good to see why 'removeFromUnrevertContext' is
choking on this particular case, even after this ticket is closed.

Uploaded a smaller (7 Mbytes) version of the forensic repo:

https://dl.dropboxusercontent.com/u/6239815/forensic2.tar.lzma

Download, uncompress, rebuild pristine (darcs repair) then run optimize
reorder before and after patch1169.

The following patch sent by Ale Gadea <alex.aegf@gmail.com> updated issue Issue2361 with
status=resolved;resolvedin=2.10.0 HEAD

* Resolve Issue2361: optimize --reorder runs forever with one repository 
Ignore-this: b18bc37e5d6668df62f05679c629e08c