Filter merged PRs on the contributors list

[deniak]

The contributors page for a given repository lists substantive PRs whether or not they are open or closed.
Only the merged PRs should be listed.

See #269 (comment)

[dontcallmedom]

For CGs that contribute to WGs recurringly (e.g. WebAssembly), reviewing substantive contributions would be best done on a time basis, each CG→WG transition serving as the starting point for a new IPR review.

This would be much easier if the listing of substantive PRs had data about the merged date as well