git: Use nvimdiff1 as the merge tool

The feature provided by the vim-diffconflicts plugin seems to have been
merged upstream. Use of nvimdiff1 should now suffice.

See https://www.eseth.org/2020/mergetools.html

There also seems to be a new proposal adding layout support which will
be interesting once merged.
https://lore.kernel.org/all/20220327112307.151044-1-greenfoo@u92.eu/

git/.gitconfig

@@ -26,15 +26,12 @@
 [difftool "kitty"]
     cmd = kitty +kitten diff $LOCAL $REMOTE
 [merge]
-	tool = diffconflicts
+	tool = nvimdiff1
 	conflictstyle = zdiff3
 	ff = only
 [mergetool]
 	prompt = false
 	keepBackup = false
-[mergetool "diffconflicts"]
-	cmd = nvim -c DiffConflicts $MERGED $BASE $LOCAL $REMOTE
-	trustExitCode = true
 [rebase]
 	autosquash = true
 	autoStash = true