Issue 3645: Allow \partial to occur in mid-piece

This is done by moving measurePosition to negative values when at the
beginning of a measure, and to a respective distance before the end of
the measure in case the measure has already been started.
Document in-piece use of \partial
Issue 355: \repeat bar lines get messed up

% the second repeat should start at the beginning of a bar, not
% continue from the incomplete bar in the first ending.

\version "2.17.30"
\new Staff \relative {
  \partial 4
  \repeat volta 4 { e' | c2 d2 | e2 f2 | }
  \alternative { { g4 g g } { a a a a } }
  a b2.
}