How to get p4merge to come up when I do git mergetool and not filemerge
It would be best to install p4merge directly from their vendor as their brew cask is not maintained by the authors. First download p4merge and drag &amp; drop to your applications from their download page: https://www.perforce.com/downloads/visual-merge-tool

After p4merge.app is under /Applications you can configure git to use p4merge as default merge and diff tool.

There is active discussion in this gist document, better check the latest developments from there https://gist.github.com/tony4d/3454372 

You can use the following configuration for Mojave.

[merge]
    tool = p4mergetool
    stat = true
[mergetool "p4mergetool"]
    cmd = /Applications/p4merge.app/Contents/MacOS/p4merge "$BASE" "$LOCAL" "$REMOTE" "$MERGED"
    keepBackup = false
    keepTemporaries = false
    trustExitCode = false
    prompt = false
[diff]
    tool = p4mergetool
    renames = copies
    mnemonicprefix = true
[difftool "p4mergetool"]
    cmd = /Applications/p4merge.app/Contents/MacOS/p4merge "$LOCAL" "$REMOTE"
    keepBackup = false
    keepTemporaries = false
    trustExitCode = false
    prompt = false