magit-move-branch: cosmetics
magit-delete-branch: restore old behaviour

When the user tries to delete the checked out branch and then replies
that she wants to checkout master first then do so *and* delete the
other branch.

Before the previous commit 2107722 that touched this code we already did
that and I believe it was not intentional to change that.
magit-git-dir: add optional SUBPATH argument
remove magit-tree-contents

It was only used once and did in 13 complicated lines
what can be achieved in one simple line.
de-obfuscate magit-list-interesting-refs
fix more section headings

* rename "Branch Utilities" to "Branch Manager Mode"
* Add "Logging (continued)"
  (should be mered with "Logging" elsewhere in the file)