magit-pull-from-upstream: don't explicitly specify remote branch
Previously we used `git pull REMOTE BRANCH' and now we just use `git pull REMOTE'. This has the advantage that tags that point to commits which are being pulled are also fetched. Users have come to expect that behavior. We did not do this previously because I considered it to be unsafe. The documentation of `git push [REMOTE]' claims that it pushes to the upstream and fails to mention the various variables that can cause it to push somewhere else. The documentation of `git pull [REMOTE]' also does not mention any such variables, but as the push documentation demonstrates, this is no guarantee that there are none. On the other hand, unlike with pull, I am not actually aware of any such variables. I have decided to drop the BRANCH argument because accidentally pulling something different from what one wanted to pull, is much less dangerous than accidentally pushing to the wrong place.
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