I think I've figured out what tortoisehg's logic is here (though I haven't checked the source to be sure).
As you've guessed, tortoise shows files changed on both sides of a merge with a double arrow. However, it does not look simply at the diff of the merge to each of its parents (e.g. p1(58eb7c70)::58eb7c70 and p2(58eb7c70)::58eb7c70). Instead, tortoise finds all changes introduced in the merge, compared the last common ancestor of the two parents.
Let's take the tortoise repo as an example. The graph view of the ancestry of 58eb7c70 is:
Jonathan:tortoisehg $ hg log --graph -r ::58eb7c70 -l 5 --template "{node|short}\n{desc|firstline}\n\n"
o 58eb7c70d501
|\ Merge with stable (noop)
| |
| o 39c95a813105
| | repowidget: show all errors on infobar
| |
| o da7ff15b4b96
| | repowidget: limit infobar error messages to 2 lines of up to 140 chars by default
| |
o | 6c716caa11fd
|\| Merge with stable
| |
| o 48c055ad634f
| | sync: show non-ascii command-line arguments correctly
| |
As you can see, merge 58eb7c70d501 merged two branches of development, with one changeset (p1, 6c716caa11fd) on one side, but two on the other (p2, 39c95a813105, and its parent, da7ff15b4b96). The point where these branches diverged is the last common ancestor of p1 and p2 -- 48c055ad634f.
(The last common ancestor can be found directly with hg log -r "last(ancestor(p1(58eb7c70), p2(58eb7c70)))")
Let's look at the changes that were made on those two branches. We'll compare each parent of the merge with the common ancestor:
Jonathan:tortoisehg $ hg status --rev "48c055ad634f::6c716caa11fd"
M .hgtags
M tortoisehg/hgqt/commit.py
M tortoisehg/hgqt/compress.py
M tortoisehg/hgqt/hgemail.py
M tortoisehg/hgqt/postreview.py
M tortoisehg/hgqt/purge.py
M tortoisehg/hgqt/rename.py
M tortoisehg/hgqt/repowidget.py
M tortoisehg/hgqt/revset.py
M tortoisehg/hgqt/run.py
M tortoisehg/hgqt/settings.py
M tortoisehg/hgqt/status.py
M tortoisehg/hgqt/sync.py
M tortoisehg/hgqt/visdiff.py
M tortoisehg/util/cachethg.py
M tortoisehg/util/hglib.py
Jonathan:tortoisehg $ hg status --rev "48c055ad634f::39c95a813105"
M tortoisehg/hgqt/repowidget.py
These are the changes that were actually merged by 58eb7c70d501 -- everything changed on the two branches since they diverged. As you can see, the only file in common between the lists -- the only file that was changed on both branches -- is tortoisehg/hgqt/repowidget.py, just as you expected. You'll see that this file was changed in da7ff15b4b96, the one changeset that's not a parent of the merge but is still included in the changes merged from the two branches.