Add PR number to commit message upon merge

[marcosfelt]

First, thanks for this amazing tool!

Currently, it is difficult to see which PR generated a commit on main. To find the PR associated with a commit, you have to look through all closed PRs and try to find the correct one.

I wonder if SPR could dynamically add the PR # to commits when syncing to GitHub. This would be similar to how GitHub adds the PR# to a squash merge or merge commit.