update: fix typo/stale comment to match code The comment about "obsolete.background" seems to have been about obsolete.foreground ever since it was introduced in a59e575c6ff8 (update: allow dirty update to foreground (successors), 2013-04-16), so let's change it.

merge: remove unused handling of default destination in merge.update() As far as I can tell, all the callers of merge.update() have been migrated over to use destutil to find the default destination when the revision was not specified. So it's time to delete the code for handling a node value of None. Let's add an assertion that node is not None, so any extensions relying on it will not silently misbehave.

update: localize logic around which ancestor to use The merge code works by applying the changes between an ancestor and the working copy onto the destination. To achieve an update, it sets the ancestor to be the parent of the working copy. To achieve a clean update (update -C), it sets the ancestor to be the working copy itself (so there are no changes to carry over). The logic for this was spread out a bit. Let's move it all to one place so it's easier to follow the reason for it. Also add some documentation.

tests: add test for updating to null revision While working on merge.py, I realized that we don't (as far as I could tell) have any tests for updating to the null revision with a dirty working copy. This adds some simple tests for that.

import: mention "stdin" (abbreviated) and add example I actually didn't even think it was possible because I searched the help text for "stdin", and didn't even think of searching for "standard input". Let's mention the abbreviated form too to help others like me. (When importing from stdin, we actually print a message saying "applying patch from stdin".) This patch also adds an example showing how to import from stdin.

merge: print status message before launching external merge tool It seems somewhat common that people run into a merge conflict and don't notice the launched merge tool, and instead they think hg just hung. Let's print a message for each file that we launch a GUI merge tool for.