Controls which merge resolution program is used by
linkgit:git-mergetool[1]. Valid built-in values are: "kdiff3",
"tkdiff", "meld", "xxdiff", "emerge", "vimdiff", "gvimdiff",
- "diffuse", "ecmerge", "tortoisemerge", "araxis", and
+ "diffuse", "ecmerge", "tortoisemerge", "p4merge", "araxis" and
"opendiff". Any other value is treated is custom merge tool
and there must be a corresponding mergetool.<tool>.cmd option.