bug#24303: Lets use git-subtree to import undo-tree into elpa

[Jonas Bernoulli]

Hello,

I wanted to address some compiler warnings in undo-tree.el but in the
process I noticed that the upstream repository and elpa are not in
sync and that upstream changes aren't merged using `git subtree'.

I think these two issues are related and would like to suggest that
`git subtree' be used from now on.

Because that isn't possible when the directory in question already
exists we temporarily have to remove it.  And we have to commit that
change because otherwise `git subtree' would refuse to do anything.

  git rm -r packages/undo-tree
  git commit -m "Temporarily remove undo-tree
  
  So that it can be added again, this time using `git subtree'."

Now we can add the subtree.  The documentation in the README suggests
that `--squash' should not be used, but there are already quite a few
packages with are importing by squashing all upstream changes and doing
it that way would result in something very similar to how it has been
done manually for this package in the past (on the surface), so I think
we might want use it here:

  git subtree add [--squash] --prefix=packages/undo-tree \
    http://www.dr-qubit.org/git/undo-tree.git master

>From now on new upstream changes can be imported like so:

  git subtree pull [--squash] --prefix=packages/undo-tree \
    http://www.dr-qubit.org/git/undo-tree.git master

Also note that in the upstream repository the "release/0.6.5" should
be created.

  Best regards,
  Jonas