I had the same problem. It seems that the postinstall sript in mercurial_1.4.1-maemo1_armel.deb deletes the folder that to where it tries to install the osutil.so (should be in /usr/lib/python2.5/site-packages/mercurial if not in there, then the next steps might help).
To fix this I did the following:

1. copy the mercurial_1.4.1-maemo1_armel.deb to computer where you have ar command in you use
2. follow the instructions here https://gist.github.com/748313/15c04...d5a8471f58e0c1 but just to be on the safe side I didn't remove the skripts but commented out the part in postinst script where it removes the files
3. copied the new deb file to the device and installed it there.
4. re-installed the mercurial-common

and that's it