Fix loss of forward repeat marks when exporting PartStaffs

[jacobtylerwalls]

Fixes #1400

Previously, the part staff exporter overzealously deduplicated <barline> tags (including child <repeat> tags).

Now, it checks that the location attribute is identical before deduplicating.

Prevents losing forward barlines as reported in #1400.

@gregchapman-dev does this resolve your issue?

[coveralls]

Coverage increased (+0.001%) to 93.078% when pulling 91cd8e6 on partStaff-repeats into 29b67ed on master.

[mscuthbert]

Thanks! Looks very good. Merging.

(This might also fix an issue with final barlines disappearing from one part in the piano scores).