Git - fatal: refusing to merge unrelated histories

During a merge between two branches, if the following error is returned:

$ git branch
* master
develop
$ git merge develop
fatal: refusing to merge unrelated histories
Error redoing merge <version-id>

Retry with the following option:

$ git merge develop --allow-unrelated-histories

This option is used in a rare event that merges histories of two projects that started their lives independently.