evolve: refactor content-divergence resolution logic
> What is the case we are looking at?
This is about refactoring the part of content-div resolution logic where it
decides which cset should be relocated and where.
> What is a "topologicial common ancestors" vs a "greatest common ancestors"?
`tca` is an ancestor which we can decide/find by looking at the at graph
visually for e.g
```
c3(*) c4(*)
| |
c2(x) c1(x)
c5 | /
\ | /
c0
```
(c5 is the successor of c2 and c1)
now here,
`tca` of c3 and c4 is: c0
`gca` of c3 and c4 is: c5
> What is the new top-level logic/behavior that makes it better?
The old code had some unnecessary edge cases just because we were using `gca`,
since it can point to a revision that is not a topological ancestor.
For e.g see b779b40f996e
Eventually, the code around this was getting messy unnecessarily. So I looked
into it and found a simple and more robust approach.
And in new code, it is simple and straightforward (and easy to understand),
where we handle the following 4 cases when solving content-div:
1) when both are on the same parent
=> (no need to do anything special, and simply proceed)
2) both are on the different parent but a) `tca` is the parent of one of them
or b) there is no non-obsolete revision between `tca` and one of the
divergent cset.
=> (relocate one to the other side and proceed)
3) both are on different parents and `tca` is not the parent of any of them and
there is at least one non-obsolete cset between tca and both the divergent
cset i.e (tca::div1) and (tca::div2) both the ranges have at least one non-obs
revision.
=> (this is the case which we don't handle yet, but the solution would be to
prompt the user to choose an evolve destination.)
4) both are in the parent-child relation
=> (both are merged and new cset will be based on the successor of `tca`)
Changes in test-evolve-issue5958.t demonstrate that new code also covered case4
because in a resolution of "two divergent csets with parent-child relation"
there should be one cset as a result and no orphan revs (as you can see there
was an orphan before this patch).
extend-diff-ignore = "^(contrib/|.gitlab-ci.yml|.hg-format-source|MANIFEST|Makefile|docs/tutorial/.netlify|hgext3rd/evolve/hack/|tests/.testtimes)"