Generating merge logs with mercurial - mercurial

Question

We're using Mercurial for source control and would like to introduce automatic merging and changelog for our QA process.
For the purpose of this discussion, let's just assume that we use a simple 3-branch flow like the below:
\ = forward merge
| = back merge of fix
Default --O--A--*--B--*---
\ ^ \
\ | \
QA -----M--X--C--*--N---
^ \
| \
Master --------H--------*---
So the question is:
Is it possible to construct a revset query that, at merge N, preferably before but otherwise after actually performing it, will result in a log of all commits on Default branch, which aren't yet in QA branch (On the above drawing that would be A and B).
Alternatively formulation:
Is it possible to construct a revset query that will return all the changesets, which would get merged if we merged Default into QA.
I'm only interested in commits on Default branch, not individual commits in feature branches already merged into Default (Feature branches not included on the drawing). I am also not interested in commits on the QA branch.
The repository is pretty well maintained, but a couple of years old and contains about 13000 nodes.
I've been experimenting with ancestor and ancestors, but either I get X as the latest common ancestor, or some very very old node. I understand the logic of getting X, but I'm really after O.
The most luck I've had is with variations of this:
hg log --rev "((heads(branch('Default')) and !closed()) % heads(branch('QA'))) and branch('Default') and p2(!branch('QA'))"
hg log --rev "((heads(branch('Default')) and !closed()) % heads(branch('QA'))) and branch('Default') and !children(branch('QA'))"
Update
Using "hg merge --preview --rev XYZ", as suggested by Tom, generates output like this:
changeset: 13070:7e59fc16aa4e
branch: Default
parent: 13068:5b9409ad504f
parent: 13069:849bd43d2023
user: *******
date: Mon Dec 18 18:40:46 2017 +0200
summary: Merged Feature branch A
changeset: 13071:07470ff919ff
branch: Feature branch B
parent: 13061:540eda2c959b
parent: 13068:5b9409ad504f
user: *****
date: Mon Dec 18 18:49:42 2017 +0200
summary: Merge with Default
changeset: 13072:a53260ffabca
branch: Feature branch B
user: *****
date: Mon Dec 18 18:58:05 2017 +0200
summary: Some text
changeset: 13073:37c895f2abf0
branch: Default
parent: 13070:7e59fc16aa4e
parent: 13072:a53260ffabca
user: *******
date: Mon Dec 18 18:58:05 2017 +0200
summary: Merged Feature branch B
Not too bad, but not super fun either. At least it requires more logic than simple grep/sls to parse.
Thanks in advance,
John

Answer 1

I found a solution that mostly does what I want:
hg log --rev "branch('Default') & !ancestors(branch('QA')) & ancestors(heads(branch('Default')) & !closed()) & !children(branch('QA'))"
The only thing now missing is to exclude multiple head merges, since they're just noise in a changelog.
Update:
So I picked this project up again and found some great answers elsewhere on SO.
The following produces output similar to hg merge --preview, but can be templated:
hg log -r "ancestors('$sourceBranch') - ancestors(.)" --template $template
This filters out all commits not on source branch:
hg log -r "(ancestors('$sourceBranch') - ancestors(.)) and (branch('$sourceBranch'))" --template $template