Install latest tortoiseSVN (tortoise merge is included).
Assumed TortoiseSVN is installed to C:\Program Files\TortoiseSVN\
Edit .hgrc
[extdiff]
cmd.tortoisemerge = C:\Program Files\TortoiseSVN\bin\TortoiseMerge.exe
[merge-tools]
tortoisemerge.args=/base:$base /mine:$local /theirs:$other /merged:$output
tortoisemerge.regkey=Software\TortoiseSVN
tortoisemerge.regkeyalt=Software\Wow6432Node\TortoiseSVN
tortoisemerge.checkchanged=True
tortoisemerge.gui=True
tortoisemerge.priority=-8
tortoisemerge.diffargs=/base:$parent /mine:$child /basename:'$plabel1' /minename:'$clabel'
[ui]
merge = tortoisemerge
[tortoisehg]
vdiff = tortoisemerge