parse_hint_target arguments

https://github.com/googlei18n/glyphsLib/blob/cfef8fb7d189c3e52ba89b6c6ddaa99a1e85ff8d/Lib/glyphsLib/classes.py#L243

`parse_hint_target()` should accept indexes with 2, 3 and 4 values. {1, 2}, {1, 2, 3}, {1, 2, 3, 4}. That happens with TrueType hints that are attached to interesting segments.

Open Lib/glyphsLib/classes.py at the parse_hint_target() definition referenced in the issue and trace how hint target indexes are parsed. Verify the behavior for two-, three-, and four-value inputs, and consider the existing parsing tests if present. Done means all three input lengths are accepted for TrueType hint targets without regressing existing cases.

Written by the indexing model from the issue text.