docs: clarify fork_point is merge-base

[9999years]

I was a bit surprised that there wasn't a merge_base function, and more surprised that searching this page for "merge-base" didn't find anything.

Let's add a note here.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
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  how it works, how it's organized), including any code drafted by an LLM.
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[joyously]

Why use a hyphen and quotes?
Why put it so far away from "common ancestor(s) of all commits in x" ?
Who uses the term "merge-base"? Should it be defined somewhere?

[9999years]

@joyously git merge-base is the Git equivalent here, so it's an obvious thing to search for (it's the base of the two commits when merging).

It also comes up in discussion and writing about Git, e.g. this question "How to find the most recent common ancestor of two Git branches?" suggests using the git merge-base command: https://stackoverflow.com/questions/1549146/git-find-the-most-recent-common-ancestor-of-two-branches

[joyously]

Perhaps that should go in the Git comparison page, as I am an example of a user that knew nothing about it.

[martinvonz]

We have this on this page under the x::y operator:

This is what git log calls --ancestry-path x..y"

So perhaps we can use similar language here, i.e. saying that it's what git merge-base calculates? Oh, we should probably also point out that it's not what git merge-base --fork-point does :)