magit-cherry-mode: don't mention magit-toggle-section

Don't mention `magit-toggle-section' in doc-string because there
is no section that could be toggled in this mode's buffer.
re-highlight section when expanding involves insertion

The bodies of some sections are not inserted until they are expanded.
In those cases we need to re-highlight section, otherwise the new text
would not be highlighted.
magit-dimmed: new face
add magit-refs-mode and magit-show-refs

Add a new mode `magit-refs-mode' which replaces the two old modes
`magit-branch-manager-mode' and `magit-wazzup-mode'.  The new entry
command is `magit-show-refs', replacing `magit-branch-manager' and
`magit-wazzup'.

Originally these had to be different modes because wazzup was so
slow that users would only tolerate it when they actually needed the
additional information it provided compared to the branch manager.
I have made wazzup fast quite some time ago but until now didn't get
around to merging the two modes.

The new mode is called "Refs Mode" because it lists all kinds of
references not just branches (that was already the case of the old
"'Branch' Manager" too).
change key bindings for magit-show-refs and magit-cherry

The old `magit-branch-manager' command was only bound in popups while
the old `magit-wazzup' command was only bound in the mode map (and the
dispatch popup).

Since there now is only `magit-show-refs', we can do better.  Because
this command is important enough give it the mode map.  But don't use
"w" which was used by `magit-wazzup' (and which we are going to use for
one of the sequence commands) instead use "y".  "y" was previously bound
to `magit-cherry', which can be considered a lightweight, but less
important, variant of the cherry-feature of `magit-show-refs', and is
now moved to "Y".

Do not bind `magit-show-refs' in any popups (except for the dispatch
popup).  Commands that are bound in the mode map don't need to be
available from popups (except the dispatch popup whose purpose is to
help users who don't know the binding in the mode map).