linkgit:git-mergetool[1]. Valid built-in values are: "kdiff3",
"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff", and
"opendiff". Any other value is treated is custom merge tool
linkgit:git-mergetool[1]. Valid built-in values are: "kdiff3",
"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff", and
"opendiff". Any other value is treated is custom merge tool