I switched from Windows to Mac for work a little over a year ago (I've been using Macs at home for years) and there is one program I still could not live without - Scooter Software's wonderful Beyond Compare. I've tried every single diff tool available and none have lived up to Beyond Compare. It truly lives up to its name.
I've had Beyond Compare running in WineBottler for a while now, but was never able to get passing arguments to work correctly. Today I had a revelation. On creating the bottle it asks you for arguments to pass the executable, I had not provided any. Ends up it then passes the first argument as an empty string, which prevented the first argument being a file! Huzzah, an easy fix, pass it an innocuous argument! So here are some directions on getting it set up!
The first step is to download WineBottler if you don't already have it. Just drag the WineBottler.app and Wine.app into your /Applications directory. Then download the Windows version of the Beyond Compare Installer and hang onto it.
Launch WineBottler.app. In the left pane choose "Create Custom Prefixes", then for the Install File, choose the Beyond Compare Installer downloaded earlier. To my knowledge no Winetricks are necessary. Check the Self-contained checkbox to make a stand alone application.
IMPORTANT: WineBottler passes a blank argument if you leave the Runtime Arguments blank, which Beyond Compare interprets as the first filename - an annoying bug that took me months to figure out. To get around this we can pass it an innocuous argument (see arguments). I chose /nobackups because I don't want backups and they are turned off by default anyway.
Click Install. It will ask you to save your App, do so in /Applications. I named mine as BeyondCompare.app and paths later will reflect this.
Follow the installer. After the installer completes Beyond Compare may pop up if you've left the launch checkbox checked. Just close the window. Choose BCompare.exe when it asks you for the default executable (BComp.exe is for batch scripting).
The paths in the following presume you named your app BeyondCompare.app so these may be adjusted as necessary.
Add the following to your ~/.gitconfig file. Remove anything existing that contradicts these settings. I've named the diff and merge tools bc3wb as git already has baked in support for the Windows version, which it calls bc3.
[diff]
tool = bc3wb
[difftool "bc3wb"]
cmd = /Applications/BeyondCompare.app/Contents/MacOS/WineBottlerStarter \"$LOCAL\" \"$PWD/$REMOTE\"
[merge]
tool = bc3wb
[mergetool "bc3wb"]
cmd = /Applications/BeyondCompare.app/Contents/MacOS/WineBottlerStarter \"$PWD/$LOCAL\" \"$PWD/$REMOTE\" \"$PWD/$BASE\" \"$PWD/$MERGED\"
keepBackup = false
trustExitCode = false
[diff]
tool = bc3wb
[difftool "bc3wb"]
cmd = /Applications/BeyondCompare.app/Contents/MacOS/WineBottlerStarter \"$LOCAL\" \"$PWD/$REMOTE\"
Note: I am unable to get the mergetool portion working in Beyond Compare 2 or 3 Standard as WineBottler is interpreting /savepath= as a filepath and converting it to z:\savepath= rather than properly passing the arguments. If anyone has any advice on how to fix this I would be ecstatic.
Voila, now you can git mergetool and git difftool it up!
You mentioned you had tried every other tool - have you tried the open source kdiff3 tool? I switched from Beyond Compare when I found that I was often having to work in environments where I couldn't get BC, but I've been happy.
Had to install XQuartz, and do an initial run of Wine before it would let me set up BeyondCompare in WineBottler. Might be helpful info for other WineBottler noobs like me.
Wow, your point regarding the empty command line argument would have taken ages for me to figure out, thanks! Couldn't figure out how to modify my existing WineBottler setup of Beyond Compare so I had to recreate the prefix a few times before I got it right, but it works now.
Hi Jesse, 
Thanks a lot. This was very usefull. 
Been trying to get this working on OSX 10.8, WineBottler 1.5.30, Beyond Compare 3.3.7.15876