I don't know why TortoiseHg does not come with this built-in, because TortoiseSVN and TortoiseGIT do.
- TortoiseIDiff.exe , TortoiseHg - Mercurial.ini, Windows ( ) > TortoiseHg > > :
[extdiff]
cmd.tortoisediff = C:\Program Files\TortoiseHg\TortoiseIDiff.exe
opts.tortoisediff = /left:"$parent" /lefttitle:"$plabel1" /right:"$child" /righttitle:"$clabel" /showinfo