The default immutable_heads() includes tags(), which makes sense, but computing
heads(tags()) can be expensive because the tags() set is usually sparse. For
example, "jj bench revset 'heads(tags())'" took 157ms in my linux stable
mirror. We can of course optimize the heads evaluation by using bit set or
segmented index, but the query includes many historical heads if the repository
has per-release branches, which are uninteresting anyway. So, this patch
replaces heads(immutable_heads()) with trunk().
The reason we include heads(immutable_heads()) is to mitigate the following
problem. Suppose trunk() is the branch to be based off, I think using trunk()
here is pretty good.
```
A B
*---*----* trunk() ⊆ immutable_heads()
\
* C
```
https://github.com/martinvonz/jj/pull/2247#discussion_r1335078879
62f0cb8c3f