Mercurial: update to a revision, but not modify local files

You can use hg debugsetparents to set the parent revision of your working copy.

You might find this article useful. The original link no longer exists, so it now references the Internet Archive's copy:

https://web.archive.org/web/20170805042314/http://hgtip.com/tips/advanced/2010-04-23-debug-command-tricks

In particular, to "reparent" your current working directory, issue:

hg debugsetparent 20
hg debugrebuildstate