How do you see / show a git merge conflict resolution that was done, given a merge commit SHA1?

If you know the ref, then git show <MERGE_COMMIT> will show you the resolution done (if any) for the merge commit.

For log, use git log -p -c or git log -p --cc. From the manpage of git log:

   -c       With this option, diff output for a merge commit shows the differences from each        of the parents to the merge result simultaneously instead of showing pairwise        diff between a parent and the result one at a time. Furthermore, it lists only        files which were modified from all parents.   --cc       This flag implies the -c option and further compresses the patch output by        omitting uninteresting hunks whose contents in the parents have only two        variants and the merge result picks one of them without modification.

Slight bikeshed: you could use diff3 or kdiff3 to see the merge in reverse, particularly if it was a (git style) 'evil' merge, where a secondary change was introduced to resolve the conflict. (watch out for an exploding head trying to see how it 'backs out' the changes;-)

Obviously the 'base' commit would be the merged commit.