[exim-dev] Git changes, viewing and pushing

Perhaps I just need more sleep than a leaking air-mattress provides, but
could a Git expert please help me with two points?

 (1) How to view which changes are in one repository but not another?
     It's acceptable for the answer to involve my learning about
     branches in Git and pulling multiple origins to local and diffing
     them.

     For seeing what's in /users/pdp/exim.git on tahini but not in the
     master, there must be a better way than looking at the shortlogs of
     both and trying to spot which changes have and have not made it in?

(2) Selectively pushing commits: how?

     Given four commits in the local tree, replicated to pdp@tahini's
     tree, *three* of which I wish to push to the master, how do I push
     those commits?

     For the remaining commit, once that's done, what's the correct way
     to get rid of it as bad work?  Nuke the repository and re-clone?
     Rollback?

Thanks,
-Phil