Make "git resolve" take the merge message in $3

It used to do "Merge $3" as the message, but that ends up being
inconvenient, and much more easily done inside git-pull-script instead.

This makes the third argument to "git resolve" much easier to explain.

git-pull-script

@@ -22,4 +22,4 @@ git-fetch-script "$merge_repo" "$merge_head" || exit 1
 git-resolve-script \
 	"$(cat "$GIT_DIR"/HEAD)" \
 	"$(cat "$GIT_DIR"/FETCH_HEAD)" \
-	"$merge_name"
+	"Merge $merge_name"