fatal: refusing to merge unrelated histories

This problem generally occur when you try to checkout repo for the first time and try to commit some files in repo.

and during that following error message is shown from git:

Problem:

fatal: refusing to merge unrelated histories
Error redoing merge 1234deadbeef1234deadbeef

Solution:
1. open cmd.exe
2. open your repo path in cmd
3. Run command 

git pull origin master --allow-unrelated-histories

this should solve your problem.