[ushi]

yes. its implemented:

  --allow-unrelated-histories

    By default, git merge command refuses to merge histories
    that do not share a common ancestor. This option can be
    used to override this safety when merging histories of
    two projects that started their lives independently. As
    that is a very rare occasion, no configuration variable
    to enable this by default exists and will not be added.

[Programmatic]

See also the relevant lines in the function he proposed a patch for: https://github.com/git/git/blob/master/builtin/merge.c#L1401

[gaius]

I have used this many times while taking the opportunity to reorganise repos while migrating earlier version control systems to Git. I still need a crib sheet every time tho’!

[jwilk]

This option was added in git v2.9.0, released on 2016-06-13.