Reverting a merge commit prevents a second merge attempt

## Description

If I merge from branch to branch, then revert the commit of the merge, I can't repeat the merge a second time without forcing a change to the branch.

## Version

* GitHub Desktop: 1.3.4

* Operating system: Microsoft Windows [Version 10.0.17134.137]

## Steps to Reproduce

1. Have branches A and B, each with separate changes
2. Select branch A as the current branch
3. Branch -> Merge into current branch... -> Branch B and click the button
4. History -> Revert the commit of the merge (e.g. there was a major problem with the merge that needs to be resolved)
5. Branch -> Merge into current branch... -> If you select Branch B, it says "nothing to merge."

### Expected Behavior

At the last step, I should not see "nothing to merge" because I reverted the previous merge, so there are, in fact, those changes still left to merge.

### Actual Behavior

I see "nothing to merge" and can't merge. To redo the merge, I have to make a change in Branch B, revert the revert, or some similar workaround.

## Additional Information

### Logs