Source: https://github.com/deadlydog/VS.DiffAllFiles/wiki/How-To-Change-The-Default-Visual-Studio-Diff-Tool-For-TFVC

Source: https://github.com/deadlydog/VS.DiffAllFiles/wiki/Common-Diff-Tool-Configurations-For-TFVC

Visual Studio Menu > Tools > Options > Source Control > Visual Studio Team Foundation Server > “Configure User Tools…”

For compare:

For merge with TortoiseGitMerge, select “Merge” by “Operation:” and as “Arguments”:  /base:%1 /mine:%2 /basename:%6 /minename:%7 /merged:%4

• %1 = Original file (in diff, the pre-changes file, in merge, the “server” or “theirs” file, the file that is the base file after “their” changes were applied)
• %2 = Modified file (in diff, the post-changes file, in merge the “yours” file – the base file with “your” changes applied)
• %3 = Base file (in the 3-way merge operation, the file which both “theirs” and “yours” are derived from – the common ancestor. This doesn’t mean it’s the version the changes were based from, since this may be a cherry-pick merge)
• %4 = Merged file (The output file for the merge operation – the filename that the merge tool should write to)
• %5 = Diff options (any additional command-line options you want to pass to your diff tool – this comes into play only when using “tf diff /options” from the command-line)
• %6 = Original file label (The label for the %1 file)
• %7 = Modified file label (The label for the %2 file)
• %8 = Base file label (The label for the %3 file)
• %9 = Merged file label (The label for the %4 file)