partCombine returns wrongly if \tags are used

[Thomas Morley]

Hi,

the following code gives bad output.

%% Start
\version "2.20.0"

one = { e''2 \tag #'score f'' \tag #'part fis'' g''1 }
two = { e''2 d'' g'1 }

\removeWithTag #'score \partCombine \one \two
%% end

Afaict, first bad commit is

commit 615571eda2393eba537c3ad0eb2d424f8dd218b0
Author: Dan Eble <nine.fierce.ballads@gmail.com>
Date:   Wed May 27 21:23:45 2015 -0400

    Issue 4423: eliminate part combiner's array of context handles (4/4)

    The Part_combine_iterator no longer creates or keeps alive the Voice
    contexts that it uses.  make-directed-part-combine-music does it.

    When the Part_combine_iterator needs to redirect a child iterator, it
    finds the new Voice by name among the siblings of the current Voice.

which is 2.19.21

Since then the code changed a lot.
As a guess, in `make-directed-part-combine-music` from
music-functions-init.ly of current master, it looks like tags are not
respected for the length of `skip`, but both are counted, thus the
length is off.

Cheers,
  Harm