Scenario: `git subtree` preserves a monorepo merge

Two lines of work under the splice, joined by a merge in the monorepo.

Monorepo (vendor/a/): cloned at seed. main and feature each committed under vendor/a/, then main was merged into feature, and feature committed again.
Upstream: seed only.

git subtree split translates every commit that affected the folder, including the side branch and merge. git-splice follows first parents from its recorded sync point instead. The merge becomes one ordinary upstream commit carrying main's content; the monorepo's private branch topology is not published.

Output

setup.bash builds this state:

$ build_scenario scenario_merge_in_monorepo

$ git log --graph --format=%s
* after the merge
*   Merge branch 'main' into feature
|\  
| * main work
* | feature work
|/  
* add vendor/a
* initial commit

git subtree split preserves the graph:

$ git subtree split --quiet --prefix=vendor/a --branch subtree-result >/dev/null && echo "created subtree-result"
created subtree-result

$ git log --graph --format=%s subtree-result | sed 's/ *$//'
* after the merge
*   Merge branch 'main' into feature
|\
| * main work
* | feature work
|/
* add vendor/a

$ git splice log
===  vendor/a (upstream has no 'feature' branch -- compared with 'main')
< 6648f65 after the merge  (Test <test@example.com>)
< 0a72413 Merge branch 'main' into feature  (Test <test@example.com>)
< 8525688 feature work  (Test <test@example.com>)

$ git splice push vendor/a
??   vendor/a: upstream has no 'feature' branch yet -- this push creates it
ok   vendor/a: pushed 6648f65 to feature

$ git -C "$UPSTREAM" log --graph --format=%s feature
* after the merge
* Merge branch 'main' into feature
* feature work
* seed

The two results contain the same files, but deliberately not the same history shape.

Output​

Output