Configure mercurial to use sourcegear diffmerge for merging

In your mercurial.ini file(i.e. $env:userprofile\mercurial.ini) specify...

[ui]
merge = vdiff

[extensions]
hgext.extdiff =

[extdiff]
cmd.vdiff = C:\Program Files\SourceGear\Common\DiffMerge\sgdm.exe

[merge-tools]
vdiff.executable = C:\Program Files\SourceGear\Common\DiffMerge\sgdm.exe
# vdiff.args = -merge -result=$output  $local $base $other
vdiff.args = -merge -result=$output -t1="Local Version" -t2=$output -t3="Other Version" -caption=$output $local $base $other
vdiff.binary = False
vdiff.symlinks = False
vdiff.gui = True
vdiff.premerge = True

If you first want to verify that path...

test-path "C:\Program Files\SourceGear\Common\DiffMerge\sgdm.exe"

Source