Generate the same background colors for annotations on every invocation of the
annotation bar, for consistent display.
Also generate the brush objects per revision only on-demand, no need to
pregenerate all of them, as often the user only looks at the annotations for
a smaller range.
Also fix the hash algorithm for VcsRevision class to generate proper hash
values for all types. Before it would have generated a value of 0 for any
string-based revision values, like e.g. used by the git vcs plugin.