unlock shallow copy of .SD when needed in joins in j

[MichaelChirico]

Closes #1926

@franknarf1 I went through a few of the examples you posted and the fix here seems to work, it would be great if you could try and stress test it a bit anyway to make sure all the bases are covered 🙇

@jangorecki / @mattdowle do we need to re-set the locked attr on.exit() for the shallow copies as well?

[codecov]

Codecov Report

Merging #3774 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3774      +/-   ##
==========================================
+ Coverage   99.41%   99.41%   +<.01%     
==========================================
  Files          71       71              
  Lines       13225    13241      +16     
==========================================
+ Hits        13148    13164      +16     
  Misses         77       77

Impacted Files	Coverage Δ
src/init.c	100% <100%> (ø)	⬆️
R/data.table.R	100% <100%> (ø)	⬆️
src/assign.c	100% <100%> (ø)	⬆️
R/setkey.R	100% <100%> (ø)	⬆️
src/utils.c	100% <100%> (ø)	⬆️
R/bmerge.R	100% <100%> (ø)	⬆️
src/subset.c	100% <100%> (ø)	⬆️

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[jangorecki]

no idea about setattr in shallow copy

[jangorecki]

We should definitely get this in 1.12.4 to clean up new vignette. @MichaelChirico

[MichaelChirico]

it's ready to go w your approval 👍