during refresh, adjust point in each window individually

Previously point was adjusted in the selected window and so all windows
visiting the current buffer ended up with the same value for point.
Now we adjust point in each window individually.
during refresh, keep same hunk selected

During refreshes try to keep the same hunk selected,
even if it changes identity.  See #1453.
make text property `invisible' rear-nonsticky

See https://github.com/magit/magit/issues/1592,
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19188, and
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=19200.