When 2 branches do not have a common Ancestor , when creating a Pull Request to merge them, Bitbucket will not display the Diff.
We will show a warning "We couldn't load the Diff" and once PR is created we will say "Nothing to merge as one of the branches were deleted" however they were not.
In the DevTools logs we will see an error:
" Error: fatal: refusing to merge unrelated histories"

Workaround is:
do the merge locally by running:
git pull origin master --allow-unrelated-histories
git merge origin origin/master
add and commit
pit push origin master

https://stackoverflow.com/questions/37937984/git-refusing-to-merge-unrelated-histories-on-rebase