Stuck at "A merge operation in progress"

I had a very similar problem with one of my projects which gave me the same TFS message in Visual Studio 2017. I was also using git. In my case I needed to manually add a comment to the commit before posting the merge. Since I had to manually merge the conflicts, a comment was needed rather than the auto-populated message that takes place on a merge without conflicts.

I'm well aware that commits need comments but if I recall correctly, VS 2015 would auto-populate the comment on a merge, even if you manually fixed any merge issues. Regardless, in VS 2017 you will need to add a commit comment when there are resolved conflicts.

When get conflicts during a pull action from remote Git repository in Visual Studio, you need to do a manual merge to choose Remote or Local version. After you click 'Accept merge', you need to commit this merge operation. You will see that like this picture below.

If you doesn't see this window, you could also commit from pending changes window. You can see that there's nothing under Changes tab. This is because you choose local version when you merge. Of cause, there's no file changes to show under Changes. But you still need to commit the merge operation.