Improve ours/theirs conflict markers for rebase, cherry-pick

Improve ours/theirs conflict markers for rebase, cherry-pick

On conflicts in rebase or cherry-pick, the conflict markers were like
this:

    <<<<<<< OURS
    a
    =======
    b
    >>>>>>> THEIRS

This is technically correct, but it could be better.

It's especially confusing during a rebase, where the meaning of
OURS/THEIRS is not obvious. The intuition is that "ours" is the commits
that "I" did before the rebase, but it's the other way around because of
the way rebase works. See various bug reports and stackoverflow
discussions.

With this change, in the case of a cherry-pick while on master, the
markers will be like this:

    <<<<<<< master
    a
    =======
    b
    >>>>>>> bad1dea Message of the commit I'm cherry-picking

In the case of a "git rebase master":

    <<<<<<< Upstream, based on master
    a
    =======
    b
    >>>>>>> b161dea Message of a commit I'm rebasing

It's not "master" because that would only be correct for the first
cherry-pick during a rebase, after that, it's master + already
cherry-picked commits.

And in the case of a "git pull --rebase":

    <<<<<<< Upstream, based on branch 'master' of git@example.org:repo
    a
    =======
    b
    >>>>>>> b161dea Message of a commit I'm rebasing

Bug: 336819
Change-Id: I1333a8dd170bb0077f491962013485efb6f2a926
Signed-off-by: Robin Stocker <robin@nibor.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>