How do I resolve a conflict after git pull?

You don't need mergetool for this. It can be resolved pretty easily manually.

Your conflict is that your local commits added a file, vision_problem_8.h, that a remote commit also created, by a rename from vignette_generator_mashed.h. If you run ls -l vision_problem_8.h* you will probably see multiple versions of this file that git has preserved for you. One of them will be yours, another of them will be the remote version. You can use an editor or whatever tools you like to resolve the conflicting contents. When you're done, git add the affected files and commit to complete the merge.

If you just want to use the remote commit's version, then you can just move the copy that you didn't write into place and git add it.

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Regarding the merge tools, have a look at git help mergetool. Basically, it's going to try running each of the included possibilities until it finds one, or use one you have explicitly configured.

I think you just forgot "-t" switch at your command line. According git help page it stands for "-t , --tool=" so it makes what you intended to.

Try:

git mergetool -t gvimdiff

Of course you may use your prefered merge tool instead of mine gvimdiff, meld is great too...