It is impossible to amend the tip to its parent's state using GUI
Created originally on Bitbucket by yruslan (Ruslan Yushchenko)
Consider following repository:
#!sh
hg init
echo "Original string" > a
hg add a
hg ci -m "Initial"
echo "Changed string" > a
hg ci -m "Changed"
echo "Original string" > a
Next, in TortoiseHG commit window try to select "Amend" option. There is no files marked as changed. If I unfilter 'clean' files and select file 'a', nothing will be amended also.
On the other hand, using Mercurial command line: hg ci --amend -m "Something" will do as it should.