I have to solve some conflict after a git pull.
$ git pull
CONFLICT (rename/add): Renamed vignette_generator_mashed.h->vision_problem_8.h in 49423dd0d47abe6d839a783b5517bdfd200a202f. vision_problem_8.h added in HEAD
Added as vision_problem_8.h~HEAD_1 instead
Removed vignette_generator_cross_square.cc
Automatic merge failed; fix conflicts and then commit the result.
So I googled it a bit, and found people saying using git mergetool. But here is what I got:
$ git mergetool
merge tool candidates: meld kdiff3 tkdiff xxdiff meld gvimdiff emerge opendiff emerge vimdiff
No files need merging
$ git mergetool opendiff
merge tool candidates: meld kdiff3 tkdiff xxdiff meld gvimdiff emerge opendiff emerge vimdiff
opendiff: file not found
So does it mean I have to install something?
What if I simply want the version from git pull to overwrite everything?
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.
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.