Subsetting altrep derived from ]]<-

[mattdowle]

Closes #3051

[codecov]

Codecov Report

Merging #3073 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3073      +/-   ##
==========================================
- Coverage   90.86%   90.83%   -0.03%     
==========================================
  Files          61       61              
  Lines       11808    11809       +1     
==========================================
- Hits        10729    10727       -2     
- Misses       1079     1082       +3

Impacted Files	Coverage Δ
src/subset.c	97.36% <100%> (+0.01%)	⬆️
src/uniqlist.c	93.61% <0%> (-1.6%)	⬇️

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