Reviving this thread, since I never quite got to my solution. Here's an 
interesting tool I found which attempts to solve this problem:

However it's designed more for rebases, and it doesn't seem to produce the 
correct (where correct === what GitHub would show) output when looking at 
two commits with a merge in between.

- Sam

On Wednesday, 22 February 2023 at 04:58:46 UTC-8 wrote:

> further example courtesy of Microsoft ... " Git history simplification can 
> be a confusing beast. "
> (ignore those '&gt' vs '>' html errors in the git code examples ;-) 
> On Wednesday, February 22, 2023 at 12:23:50 PM UTC Philip Oakley wrote:
>> I haven't tried to follow that example properly yet.. 
>> However one other thing to look at is the "History Simplification" that 
>> includes parent re-writing that's in the rev-list-options.txt file and then 
>> included in a number of man pages (log, show, short-log,..).  There are 
>> some slippery concepts in there, often context dependent!
>> On Wednesday, February 22, 2023 at 5:00:51 AM UTC 
>> wrote:
>>> Thank you both for getting back to me. The discussion in the docs about 
>>> flattening was really interesting!  I should note that the git clone / git 
>>> log command pair I provided gives me almost exactly what I want, but I need 
>>> to combine the diffs. It seems to contain the correct changes, and the 
>>> speed is pretty good too.
>>> Let me give an example of the situation I am optimizing for. I apologize 
>>> in advance I am going to use GitHub terms which I know are not pure git, 
>>> but in the end my question is a git question. 
>>> Say you're a developer working in a many-developer repository. Here's 
>>> the sequence:
>>>    - On Day 0 you check out "main" and create "my-topic-branch". You 
>>>    add commits A, B, C, D to that branch. 
>>>    - Now you open a pull request on GitHub asking to merge your branch 
>>>    "my-topic-branch" into "master". 
>>>    - You see a collaborator has landed a change to "main" since you 
>>>    started. So you do "git fetch origin main && git merge main" and make a 
>>>    merge commit in your branch. 
>>>    - Then you add three more commits E, F, G on top of that and push 
>>>    your branch again. So you have: A, B, C, D, (merge main), E, F, G.
>>>    - A coworker has already looked at commits A, B, C and wants to see 
>>>    what you've done then. So they ask GitHub to show the diff from commits 
>>> D 
>>>    through G (including the merge).
>>> When you do this, GitHub does something which (to me, anyway) is pretty 
>>> magical. You are shown only the changes that you committed to your branch 
>>> in D, E, F, and G. Changes which you merged in, which may or may not 
>>> involve the files in your Pull Request, are not shown at all since they're 
>>> not "yours".
>>> Here's a public example showing a team using this pattern. This one has 
>>> multiple merges, so I may need to find a cleaner example but hopefully this 
>>> makes sense.
>>>    - Consider this PR: 
>>>       - This is the full diff (according to GitHub) and we can see 
>>>       exactly one added line in
>>>    - Here's a merge commit: 
>>>       - We can see that on the base branch, "master", a line was added 
>>>       to the *end* of the file. There is no such addition 
>>>       displayed in the full diff.
>>>    - Here's a "magic" diff where I selected three commits (before 
>>>    merge, merge, and after merge): 
>>>       - We can see that the changes from the merge commit are not shown 
>>>       as additions! But they are present as context lines.
>>> I need to find a sequence of git commands to produce the same exact diff 
>>> that GitHub produces (and ideally do it very quickly even in a large 
>>> repository) and I just can't figure it out.
>>> Thanks,
>>> Sam
>>> On Tuesday, 21 February 2023 at 14:12:48 UTC-8 wrote:
>>>> This may also be an issue of the History Simplification process and / 
>>>> or the 'flattening' processes for history linearisation and rebases.
>>>> The flattening is a known phenomena and was currently being mentioned 
>>>> on the Git List, so I have noted this there. 
>>>> [1] 
>>>> <>
>>>> There is a technical discussion of flattening in the docs at 
>>>> Do note the original email title  "Pull is mostly evil" ;-) (whole 
>>>> thread at 
>>>> <>)
>>>> Clarifying the " excluding merge commit changes" (or misunderstandings 
>>>> if you've there were some..) would be really useful. The existing devs do 
>>>> have the 'curse of knowledge' so often can't see the problems.
>>>> On Tuesday, February 21, 2023 at 5:29:36 PM UTC Konstantin Khomoutov 
>>>> wrote:
>>>>> On Mon, Feb 20, 2023 at 09:27:20PM -0800, 'Samuel Stern' via Git for 
>>>>> human beings wrote: 
>>>>> > This is an *extremely* specific question which I've been trying to 
>>>>> get an 
>>>>> > answer to for quite a while now, so hopefully someone here knows the 
>>>>> answer. 
>>>>> > 
>>>>> > Let's say I am starting from nothing, an empty directory on a 
>>>>> server. I 
>>>>> > have: 
>>>>> > 
>>>>> > - The URL for a public git repository 
>>>>> > - Two endpoint SHAs (commits on the same branch) 
>>>>> > 
>>>>> > I want to get the complete diff between those commits *excluding* 
>>>>> merge 
>>>>> > commit changes, and I want to do this as fast as possible (so much 
>>>>> faster 
>>>>> > than cloning everything and diffing). 
>>>>> > 
>>>>> > I am able to get almost there with the following sequence: 
>>>>> > 
>>>>> > # Fast clone 
>>>>> > git clone --verbose --no-checkout --filter=blob:limit=250k 
>>>>> --single-branch 
>>>>> > --branch=${branch} --depth=${depth} $REPO_URL 
>>>>> > 
>>>>> > # Get a series of patches 
>>>>> > git log --no-merges --first-parent --patch ${base.sha}..${head.sha} 
>>>>> > 
>>>>> > However I need to get a *single* patch that represents all the 
>>>>> changes 
>>>>> > combined, not a series of patches from the log. 
>>>>> Isn't mere 
>>>>> git diff ${head.sha} ${base.sha} 
>>>>> is what you're looking for? 
>>>>> Otherwise, I'm with Philipp in that your statements (rephrased) 
>>>>> - I want to get a single combined change ("patch") describing the 
>>>>> literal 
>>>>> set of changes between such and such commits. 
>>>>> - I want changes brought in by merge commits excluded. 
>>>>> Contradict each other: I could in principle envision some algorithm 
>>>>> which 
>>>>> would try to incrementally produce a diff as in walks a chain of 
>>>>> commits and 
>>>>> tries to ignore the changes introduced by merge commits located in 
>>>>> that chain, 
>>>>> but leaving aside the fact such an algotithm would be very brittle for 
>>>>> any 
>>>>> real-world cases, I simply see no use for it - even a theoretical one. 
>>>>> You might got trapped by the fact you have found `git log` first in 
>>>>> your 
>>>>> search, and this command traverses all individual commits in the 
>>>>> subgraph it's 
>>>>> told to traverse - including "sidelines" brought in by merge commits. 
>>>>> Instead, plain old `git diff` does not traverse anything: it takes two 
>>>>> states 
>>>>> of the project and compares them. 

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