How to use Visual Studio (vsdiffmerge) as external diff tool in SourceTree for Mercurial?

For Visual Studio 2015

Put this line in the Diff Command box:

C:/Program Files (x86)/Microsoft Visual Studio 14.0/Common7/IDE/vsDiffMerge.exe

And this one in the Arguments box:

$REMOTE $LOCAL $BASE $MERGED //m

For Visual Studio 2019 Enterprise vsDiffMerge.exe location is:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\Common7\IDE\CommonExtensions\Microsoft\TeamFoundation\Team Explorer\vsDiffMerge.exe

1. You have a lot of useless (at least) blackslashes in commands and arguments
2. Read Setting Up DiffMerge page, second chapter and fix own mistakes