In git branches are lightweight. You can think of a branch as of just a label or a pointer to a particular commit X. When you "commit to a branch", it associates the old tip commit with the new commit with a parent-child relationship, and advances the branch pointer. The relationship is stored in the commit, it is separate from the branch label existence.
Each commit might have multiple parents. This is what happened when you merged: it made a new commit M with 2 parents (called "merge commit"). At that point in time (if it was done right) you probably had your "master" branch pointing to the merge commit M, and your "develop" branch still pointing to your latest dev commit (I assume it is 83bebd6 if the blue branch is "develop").
Now, to find out if your branches diverge or not, you can run:
git show master
git show develop
This will show a commit that each branch is pointing at (again think in terms of labels to commits).
To know which branch you are currently at, you run git branch
. If you are currently at "master", it means that each commit will advance and update the master
"label" (and it will "diverge" from whatever other branches you have as soon as you commit, because normally you can't commit to multiple branches at a time).
Although it's a dangerous and destructive operation, you can always "reset" your branch "label" to point to some other (previous or even totally unrelated) commit:
git checkout develop
git reset --hard 83bebd6
This will locally make so that "develop" points to commit 83bebd6.
If you want this branch to reset on the bitbucket server (and this is even more dangerous and destructive), you do:
git push -f