travis git diff behaves strangely for pull requests based on old common commits

[poeschlr]

Our syntax that should ensure only additions by the contributor are checked seems to fail lately. Travis git diff with three dots gets the same result as with two dots. If i do the same locally then three dots gets us what we want to see and two dots behaves like travis does.

See:

• Travis check regression ? kicad-footprints#1384

effected footprints (examples):

• Added Keystone 3002 kicad-footprints#1183
• added ST_VL53L0X and ST_VL53L1X kicad-footprints#804

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For reference this is how git diff should behave:

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This states that triple dot diff does not always work nicely could contain hints:
https://stackoverflow.com/questions/51618951/git-diff-check-shows-changes-of-unrelated-branch-files