$ git revert -m 1 M
-After the develpers of the side branch fixes their mistakes, the history
+After the developers of the side branch fix their mistakes, the history
may look like this:
---o---o---o---M---x---x---W---x
/ \ /
---A---B A'--B'--C'
-where Y is the revert of W, A' and B'are rerolled A and B, and there may
+where Y is the revert of W, A' and B' are rerolled A and B, and there may
also be a further fix-up C' on the side branch. "diff Y^..Y" is similar
to "diff -R W^..W" (which in turn means it is similar to "diff M^..M"),
and "diff A'^..C'" by definition would be similar but different from that,
revert of a merge was rebuilt from scratch (i.e. rebasing and fixing,
as you seem to have interpreted), then re-merging the result without
doing anything else fancy would be the right thing to do.
+ (See the ADDENDUM below for how to rebuild a branch from scratch
+ without changing its original branching-off point.)
However, there are things to keep in mind when reverting a merge (and
reverting such a revert).
ready yet, and I really need to undo _all_ of the merge"). So then you
really should revert the merge, but when you want to re-do the merge, you
now need to do it by reverting the revert.
+
+ADDENDUM
+
+Sometimes you have to rewrite one of a topic branch's commits *and* you can't
+change the topic's branching-off point. Consider the following situation:
+
+ P---o---o---M---x---x---W---x
+ \ /
+ A---B---C
+
+where commit W reverted commit M because it turned out that commit B was wrong
+and needs to be rewritten, but you need the rewritten topic to still branch
+from commit P (perhaps P is a branching-off point for yet another branch, and
+you want be able to merge the topic into both branches).
+
+The natural thing to do in this case is to checkout the A-B-C branch and use
+"rebase -i P" to change commit B. However this does not rewrite commit A,
+because "rebase -i" by default fast-forwards over any initial commits selected
+with the "pick" command. So you end up with this:
+
+ P---o---o---M---x---x---W---x
+ \ /
+ A---B---C <-- old branch
+ \
+ B'---C' <-- naively rewritten branch
+
+To merge A-B'-C' into the mainline branch you would still have to first revert
+commit W in order to pick up the changes in A, but then it's likely that the
+changes in B' will conflict with the original B changes re-introduced by the
+reversion of W.
+
+However, you can avoid these problems if you recreate the entire branch,
+including commit A:
+
+ A'---B'---C' <-- completely rewritten branch
+ /
+ P---o---o---M---x---x---W---x
+ \ /
+ A---B---C
+
+You can merge A'-B'-C' into the mainline branch without worrying about first
+reverting W. Mainline's history would look like this:
+
+ A'---B'---C'------------------
+ / \
+ P---o---o---M---x---x---W---x---M2
+ \ /
+ A---B---C
+
+But if you don't actually need to change commit A, then you need some way to
+recreate it as a new commit with the same changes in it. The rebase commmand's
+--no-ff option provides a way to do this:
+
+ $ git rebase [-i] --no-ff P
+
+The --no-ff option creates a new branch A'-B'-C' with all-new commits (all the
+SHA IDs will be different) even if in the interactive case you only actually
+modify commit B. You can then merge this new branch directly into the mainline
+branch and be sure you'll get all of the branch's changes.
+
+You can also use --no-ff in cases where you just add extra commits to the topic
+to fix it up. Let's revisit the situation discussed at the start of this howto:
+
+ P---o---o---M---x---x---W---x
+ \ /
+ A---B---C----------------D---E <-- fixed-up topic branch
+
+At this point, you can use --no-ff to recreate the topic branch:
+
+ $ git checkout E
+ $ git rebase --no-ff P
+
+yielding
+
+ A'---B'---C'------------D'---E' <-- recreated topic branch
+ /
+ P---o---o---M---x---x---W---x
+ \ /
+ A---B---C----------------D---E
+
+You can merge the recreated branch into the mainline without reverting commit W,
+and mainline's history will look like this:
+
+ A'---B'---C'------------D'---E'
+ / \
+ P---o---o---M---x---x---W---x---M2
+ \ /
+ A---B---C