Issue 5919 Make InstrumentName.X-offset more robust

Issue 5919 Make InstrumentName.X-offset more robust

Currently system-start-text::calc-y-offset takes right end of
system-start-grobs into account, which may lead to bad results, if
one of them is omitted (collapse-height) or moved to the right.
Instead we always use the value of indent for the right end and
calculate the left end similar as before.

[Dan Eble, 2020-04-18 18:38:31]

On 2020/04/18 18:14:33, thomasmorley651 wrote:

I ran make check and didn't see any differences.  Without test coverage, someone
could easily break this again without knowing it.

[dan_faithful.be, 2020-04-18 18:50:29]

On Apr 18, 2020, at 14:38, nine.fierce.ballads@gmail.com wrote:
> 
> On 2020/04/18 18:14:33, thomasmorley651 wrote:
> 
> I ran make check and didn't see any differences.  Without test coverage,
> someone could easily break this again without knowing it.

Oops.  *I* wrote that; I just neglected to delete the automatic attribution
line.
— 
Dan

[thomasmorley651, 2020-04-18 20:03:01]

Dan's advice, adds regtest