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.

[hariom-qure]

For rebase and merge: Would it make sense to interactive rebase and add the merging PR number to the summary of all commits? (like reword approximately, although we would need to add a flag to reword? or create a new editor cli?)

I'm assuming that for squash and merge, it suffices to simply add the number to summary in MergePullRequest, but the above approach has the same effect anyways

Would love to get some feedback for working on this, thanks!