Question: I am currently using TortoiseHg (Mercurial) and accidentally committed an incorrect commit message. How do I go about editing this commit message in the repository?

Answer: 
In TortoiseHg, right-click on the revision you want to modify. Choose Modify History->Import MQ. That will convert all the revisions up to and including the selected revision from Mercurial changesets into Mercurial Queue patches. Select the Patch you want to modify the message for, and it should automatically change the screen to the MQ editor. Edit the message which is in the middle of the screen, then click QRefresh. Finally, right click on the patch and choose Modify History->Finish Patch, which will convert it from a patch back into a change set.

Oh, this assumes that MQ is an active extension for TortoiseHG on this repository. If not, you should be able to click File->Settings, click Extensions, and click the mq checkbox. It should warn you that you have to close TortoiseHg before the extension is active, so close and reopen.
A little gem in the discussion above - thanks to @Codest and @Kevin Pullin. 
In TortoiseHg, there's a dropdown option adjacent to the commit button. Selecting "Amend current revision" brings back the comment and the list of files. SO useful.