Mercurial supports external diff tools with the Extdiff extension. Enable it by adding an entry to extensions in your .hgrc.

extdiff =

You can then run hg extdiff -p difft (assumes the difft binary is on your $PATH).

You can also define an alias to run difftastic with hg. Add the following to your .hgrc to run difftastic with hg dft.

cmd.dft = difft
opts.dft = --missing-as-empty

hg log -p

Mercurial does not have a way of changing the default diff tool, at least to the author's knowledge.

If you just want to view the diff of the most recent commit, you can use the following.

GIT_PAGER_IN_USE=1 hg dft -r .^ -r . | less

This is equivalent to hg log -l 1 -p, although it does not show the commit message.