git-merge: a bit more readable user guidance.

We said "fix up by hand" after failed automerge, which was a big
"Huh?  Now what?".  Be a bit more explicit without being too
verbose. Suggested by Carl Worth.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge.sh

@@ -335,5 +335,5 @@ Conflicts:
 	then
 		git-rerere
 	fi
-	die "Automatic merge failed; fix up by hand"
+	die "Automatic merge failed; fix conflicts and then commit the result."
 fi