The goal here is to allow something like "hg fast-export | git fast-import" to show a proper progress bar.
At the moment every write via ui.write and friends will hide the progress bar and re-enable it, even when stdout doesn't affect the terminal state. This would also make sense for operations like "hg diff", e.g. when writing a large/complicated diff to a file.

Will be implemented properly.

For the NetBSD repository, a trivial test with the new cache:

Friendly ping

Adjusted for most of the review comments. Added a test that books are left alone, since they will not be active in the interesting cases. I'm not touching any existing phases, so I'm not sure if I should do anything about phase testing?

It's used in reportphasechanges and that accidently turned O(n) into O(n^2). I noticed it when trying a clone --narrow of the NetBSD repo, since it hung for quite a while in that loop.

In D3439#58405, @martinvonz wrote:

Do you think we should call it commonpath() or commondir() instead in case we want commonprefix() to work for any string in the future (and not care about path separators)?

If we want to go with a --close flag, I think the most natural place would actually be hg heads. This is not really a branch operation after all, but about cutting off heads. At the same time, none of those options would allow specifying a commit message naturally.

It doesn't seem to fit the argument schema for hg branch at all. I'm open for better places, but I couldn't think of a consistent place.