Do not delete branch if it is a base branch of another PR

[hkdobrev]

I think this is separate case from #6 and arguably it could be more common.

Let's say we have branch master, feature-x and feature-y (branched from feature-x).

And we have the following pull requests:

• PR 1:
  Base: master
  Head: feature-x

• PR 2:
  Base: feature-x
  Head: feature-y

Then if we merge PR 1 first before merging PR 2 and before changing the base on PR 2, GitHub does not show you the "Delete branch" option in the UI on PR 1.

However, this GitHub action does not check for that and through the API manages to delete the branch.

This results in closing PR 2 and if you then force-push into feature-y, you can change the base to master, but GitHub does not allow even reopening this PR again after it was force-pushed.

My suggestion is to follow the GitHub UI and do not delete a branch when it is a base branch for another PR, even if the PR where this branch is the head of the PR is merged.

I believe we could check this through a GitHub API call. I could try submitting a patch, but I wanted to get some feedback first.

[hkdobrev]

Here's a diagram:

       ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐            ┌ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ┐
                PR 1                              PR 2
       ▼                   │            ▼                       │
┌────────────┐             ┌────────────┐                ┌────────────┐
│   master   │             │ feature-x  │                │ feature-y  │
└────────────┘             └────────────┘                └────────────┘
      •◀────────────•◀────────────•◀───────────────•◀───────────•

So if PR 2 is opened before merging PR 1 and PR 2 is still open at the time PR 1 is merged, feature-x should not be deleted when PR 1 is merged.

[hkdobrev]

Any comments? Anyone having the same problem? Thanks!