git-hg seems don't respect the order of a merge
In Git, usually, the first parent of a merge is the branch which other branches merged into.
I have tried your git-hg and abourget's git-hg-again both on mutt's hg repo, it seems that the later respects the order of a merge well, the former is a bit random. You can see from the screenshots below.
- A merge history graph of mutt imported by your git-hg:

- A merge history graph of mutt imported by abourget's git-hg-again:

- The actuall history graph plotted by hgk on mutt's hg repository:

As you can see from the above, the second graph by abourget's git-hg-again is very close to the original hgk graph and is actually reflecting the real workflow of the mutt.
This is also posted as an answer to this question.
This is interesting. git-hg is really a thin layer of convenience on top of http://github.com/frej/fast-export, although the version I was using was a little over 1 year old at this point. I just updated to the latest version and I'm curious if this fixes the issue. Otherwise it should probably be reported as an issue on fast-export.