The default behavior has changed since Git 2.9:
“git merge” used to allow merging two branches that have no common
base by default, which led to a brand new history of an existing
project created and then get pulled by an unsuspecting maintainer,
which allowed an unnecessary parallel history merged into the
existing project. The command has been taught not to allow this by
default, with an escape hatch –allow-unrelated-histories option
to be used in a rare event that merges histories of two projects
that started their lives independently.
See the Git release changelog for more information.
You can use –allow-unrelated-histories to force the merge to happen.
In my case, the error was just fatal: refusing to merge unrelated histories on every try, especially the first pull request after remotely adding a Git repository.
Using the –allow-unrelated-histories flag worked with a pull request in this way:
git pull origin branchname –allow-unrelated-histories
As per 2.9.0 release notes – git pull has been taught to pass the –allow-unrelated-histories option to underlying git merge