user-manual: minor rewording for clarity.

Junio screwed up when applying the previous round of the patch;
rewording from "previous" to "old" does make the description
clearer.

Also revert the rewording from head to branch.  The description
is talking about the branch's tip commit and using the word head
is clearer.

Based on input from Sergei and Bruce.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -1367,7 +1367,7 @@ If you make a commit that you later wish you hadn't, there are two
 fundamentally different ways to fix the problem:
 
 	1. You can create a new commit that undoes whatever was done
-	by the previous commit.  This is the correct thing if your
+	by the old commit.  This is the correct thing if your
 	mistake has already been made public.
 
 	2. You can go back and modify the old commit.  You should
@@ -1567,7 +1567,7 @@ old history using, for example,
 $ git log master@{1}
 -------------------------------------------------
 
-This lists the commits reachable from the previous version of the branch.
+This lists the commits reachable from the previous version of the head.
 This syntax can be used with any git command that accepts a commit,
 not just with git log.  Some other examples: