Author of merge commit from a pull request is inconsistent between Git & Hg

Behavior in GIT:
user A codes and creates a pull request
user B reviews and clicks merge
-> generates a new merge-commit from user A

Behavior in Mercurial
user A codes and creates a pull request
user B reviews and clicks merge
-> generates a new merge-commit from user B

The correct behavior should be the Mercurial one-- however, we are showing the wrong person when the user clicks 'Merge' for Git.

PR notification emails report incorrect users as well