Issue 5621: Improve rehearsal mark position at beginning of staff. (issue 553290043 by address@hidden)

[nine . fierce . ballads]

I see many cases in the regression tests that this change fails to
improve, for example:

In input/regression/rehearsal-mark-formatters.ly, the new position of
the "H" mark is an improvement, but the new position of the "CC" mark
isn't.

In input/regression/rehearsal-mark-align-priority.ly, the marks over the
F clef didn't need to be shifted, and the mark over the G clef didn't
benefit from being shifted.

https://codereview.appspot.com/553290043/