解决办法 在 命令后加： –allow-unrelated-histories
出现错误的原因：文档
* "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.
* "git pull" has been taught to pass the "--allow-unrelated-histories"
option to underlying "git merge".
