Git for Windows: Set up Helix Visual Merge Tool (P4Merge) as default mergetool

.gitconfig

[merge]
    keepBackup = false
    tool = p4merge
[mergetool]
    prompt = false
[mergetool "p4merge"]
    cmd = p4merge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"
    keepTemporaries = false
    trustExitCode = false
    keepBackup = false
[diff]
    tool = p4merge
[difftool]
    prompt = false
[difftool "p4merge"]
    cmd = p4merge \"$LOCAL\" \"$REMOTE\"
    keepTemporaries = false
    trustExitCode = false
    keepBackup = false

HOWTO.md

Open git-bash and type

git config --global merge.tool p4merge
git config --global mergetool.p4merge.cmd "p4merge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\""
git config --global mergetool.p4merge.trustExitCode false
git config --global mergetool.keepBackup false
git config --global diff.tool p4merge
git config --global difftool.p4merge.cmd "p4merge \"$LOCAL\" \"$REMOTE\""

The .gitconfig should look like this