Use p4merge for git diff and merge on mac (update based on p4merge for mac 2018)

(update based on mac P4Merge 2018)

Assuming p4merge is already installed on mac in the default /Application folder.

1. create a sh script file called mymerge.sh with below content in /usr/local/bin folder
#!/bin/sh 
/Application/p4merge.app/Contents/Resources/launchp4merge $*


2. create a sh script file: /usr/local/bin/extMerge with the following content
#!/bin/sh mymerge.sh $*

3.create another script file: /usr/local/bin/extDiff with the following content

#!/bin/sh
[ $# -eq 7 ] && /usr/local/bin/extMerge "$2" "$5"

4. set the execute permission for the above files
$ sudo chmod +x /usr/local/bin/mymerge.sh

$ sudo chmod +x /usr/local/bin/extMerge
$ sudo chmod +x /usr/local/bin/extDiff




5. edit .gitConfig file under the user home directory as below

[merge]
  tool = extMerge
[mergetool "extMerge"]
  cmd = extMerge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"
  trustExitCode = false
  prompt = false
[diff]
  external = extDiff
  tool = extDiff
[difftool "extDiff"]
    cmd = mymerge.sh \"$LOCAL\" \"$REMOTE\"
    trustExitCode = false
    prompt = false

6. run 

git diff

or 

git difftool

or 

git mergetool 

on an updated repository to see the result.

Link:
https://community.perforce.com/s/article/2848

http://git-scm.com/book/en/Customizing-Git-Git-Configuration