Re: Commiting changes onto more than one branch

[Junio C Hamano <gitster@pobox.com>]

Mike Jarmy <mjarmy@gmail.com> writes:

> I guess I didn't explain it too well, I made it sound like v3, v4 and
> v5 were still more-or-less the same.  What I'm thinking about here is
> a case where we have switched to git a while back, and then done a lot
> of work on the various different branches, so that v3, v4 and v5 have
> diverged very far from each other.  In that case, we would never want
> to merge them together.

I take it to mean that even though v[345] have diverged, the area that the
particular change you have in mind has to touch haven't changed since they
forked.  Otherwise you wouldn't be able to apply the same change to all of
them in the same form and instead would be making the logically same
change in physically different ways per branch, and you won't be asking
this question.

        o--o--o--o--o--o--o--o v4
       /
    --o--x--x--x--x--x--x--x v3

If you implemented your change at the tip of v3 branch and merge the
result to v4, you will pull _all_ of 'x' into v4 that may not be desirable
if the branches diverged a lot, of course.  

        o--o--o--o--o--o--o--o-------M v4
       /                            /
    --o--x--x--x--x--x--x--x       /
                            \     /
                             Y---Y your change(s) == v3

But nobody tells you to do this.

Instead, you can fork such a topic from the latest common.

        o--o--o--o--o--o--o--o v4
       /
    --o--x--x--x--x--x--x--x v3
       \
        Y---Y
            your change(s)

and merging this to v3 and v4 will have the desired effect.

        o--o--o--o--o--o--o--o---------M v4
       /                              /
    --o--x--x--x--x--x--x--x---M v3  /
       \                      /     /
        Y---Y----------------.-----.
            your change(s)

The merges will incorporate this particular change alone without dragging
anything else.  When you merge it to v4, none of the unrelated 'x' will be
merged into it.

In general you shouldn't fork a topic from the _tip_ of the oldest branch,
if the branches are not meant to be merged as a whole.

Of course, if this becomes cumbersome, you would adopt a better branch
management to keep the numbers of 'x' that shouldn't be in later branches
to the minimum.