When I met a conflict, I tried to use git-mergetool to solve it. I typed:
>git mergetool -t vimdiff
It opened vimdiff in 4-way, not 3-way. My split windows in vimdiff look like:
:ls
1 #a "Gemfile.lock" line 1
2 %a "Gemfile.lock.LOCAL.4828.lock" line 1
3 a "Gemfile.lock.BASE.4828.lock" line 0
4 a "Gemfile.lock.REMOTE.4828.lock" line 0
What are they? I want a 3-way diff: target file, merge file and working file. How should I configure my git or vimdiff?
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 "Gvdiffsplit!" "$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.