rbindlist() now returns correct idcols for lists with different length vectors

[shrektan]

Closes #3785

After the PR the below code works.

library(data.table)
x = 1:1000
X = rbindlist(lapply(x, function(.) list(., 1:2, 2:3)), idcol = 'TAG')
all.equal(X$TAG, rep(x, each = 2))

x = setNames(x, paste0("nm_", x))
X = rbindlist(lapply(x, function(.) list(., 1:2, 2:3)), idcol = 'TAG')
all.equal(X$TAG, rep(names(x), each = 2))

[jangorecki]

LGTM

[codecov]

Codecov Report

Merging #3786 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3786   +/-   ##
=======================================
  Coverage   99.41%   99.41%           
=======================================
  Files          71       71           
  Lines       13245    13245           
=======================================
  Hits        13168    13168           
  Misses         77       77

Impacted Files	Coverage Δ
src/rbindlist.c	100% <100%> (ø)	⬆️

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