import: --no-commit should update .hg/last-message.txt
The patch parser goes through all of that trouble extracting the
commit message from the patch file. It seems such a waste not to
use it.
http://mercurial.selenic.com/bts/issue433
$ hg init
$ echo a > a
$ hg commit -Ama
adding a
$ hg parents -r 0 doesnotexist
abort: 'doesnotexist' not found in manifest!
[255]