Better list assign

[mattdowle]

Follow up to #3457
Satisfies the original expectation in #950

[codecov]

Codecov Report

Merging #3504 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3504      +/-   ##
==========================================
+ Coverage   96.61%   96.62%   +<.01%     
==========================================
  Files          65       65              
  Lines       12255    12258       +3     
==========================================
+ Hits        11840    11844       +4     
+ Misses        415      414       -1

Impacted Files	Coverage Δ
src/assign.c	100% <100%> (ø)	⬆️
src/uniqlist.c	96.68% <0%> (+0.55%)	⬆️

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[codecov]

Codecov Report

Merging #3504 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3504      +/-   ##
==========================================
+ Coverage   96.61%   96.66%   +0.04%     
==========================================
  Files          65       65              
  Lines       12255    12280      +25     
==========================================
+ Hits        11840    11870      +30     
+ Misses        415      410       -5

Impacted Files	Coverage Δ
src/fread.c	98.5% <100%> (ø)	⬆️
R/data.table.R	97.64% <100%> (ø)	⬆️
src/assign.c	100% <100%> (ø)	⬆️
src/uniqlist.c	98.98% <100%> (+2.85%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[mattdowle]

@MichaelChirico The merge from master looks odd in this PR. I either do a 'git merge master' locally and push, or a 'resolve conflicts' using the GitHub GUI. Both ways show as a single "Merge branch 'master' into better_list_assign" (theres a few above). But the latest merge master (that you did?) shows each commit to master separately (other PRs) and the 'files changed' tab doesn't match what I see locally; e.g. a binary file added in master from another PR is here in this PR but shows as a diff and there's no 'refresh' at the top so it's not that the diff tab hasn't updated yet.
I'll go ahead and merge. But for completeness just wondering what you did.

[MichaelChirico]

on PR branch I ran git pull origin master, i thought it was the same as git merge master 🤔 will stick w the latter now