Why does git mergetool open 4 windows in vimdiff? (I'd expect 3)

As an alternative, have you thought about using fugitive?

I'm not going to lie to you; fugitive.vim may very well be the best Git wrapper of all time.

There is a an excellent vimcast, Fugitive.vim - resolving merge conflicts with vimdiff, by Drew Neil. This is part of a series on fugitive.

The Vimcasts website is a good place to learn more about vim.

To use fugitive as you mergetool you can use the following.

git config --global mergetool.fugitive.cmd 'vim -f -c "Gdiff" "$MERGED"'
git config --global merge.tool fugitive

Note: you may want to change vim to mvim or gvim.

Fugitive has a lot more to offer than just being a merge tool script so make sure you read the documentation and/or check out the vimcasts.

After lots of research for issuing mergetool with vimdiff and only 3 windows, I came up with this configuration, that allows me to chose when I want 3 windows or the default 4 windows:

git config --global merge.tool vimdiff
git config --global alias.mt mergetool

git config --global mergetool.merge3.cmd 'vim -d -c "wincmd l" "$LOCAL" "$MERGED" "$REMOTE"'
git config --global alias.m3 'mergetool -t merge3'

Now you can start 3 windows just typing:

git m3

And the default (4 windows) will still works as expected with:

git mt

Also, you probably would like to add this lines to the end of your ~/.vimrc or /etc/vim/vimrc

 " shortcuts to vimdiff
 let mapleader=','
 let g:mapleader=','

 if &diff
    map <leader>1 :diffget LOCAL<CR>
    map <leader>2 :diffget BASE<CR>
    map <leader>3 :diffget REMOTE<CR>
 endif

This will create shortcuts like ,1 to grab from left, ,3 to grab from right (in both modes) and also ,2 to grab from base (center window) in the 4 windows mode.

That helps a lot!

My ~/.gitconfig file looks like this:

[user]
        name = Dr Beco
        email = my@email
[merge]
        tool = vimdiff
[mergetool "merge3"]
        cmd = vim -d -c \"wincmd l\" \"$LOCAL\" \"$MERGED\" \"$REMOTE\"
[alias]
        lo = log --pretty=format:\"%h %ce %cd %s\" --graph
        co = checkout
        ci = commit
        cm = commit -a -m
        st = status
        br = branch
        m3 = mergetool -t merge3
        mt = mergetool
[diff]
        tool = vimdiff

I hope this helps you (and those who get to here).

Modifying a bit the commands from this page:

git config --global mergetool.vimdiff3.cmd 'vim -f -d "$LOCAL" "$MERGED" "$REMOTE"'
git config --global merge.tool vimdiff3

• 'Merged' would be your working copy.
• 'Local' the file that is in the branch you are trying to make the changes
• 'Remote' the file from the branch you are trying to merge with.

And then you execute the command: git mergetool.

Note: I use fugitive also and highly recommend it.