Shallow bugfix

[MarkusBonsch]

Closes #2336 by changing the behaviour of .shallow as follows:

• if not all columns of a secondary index are copied and the index has length(idx) > 0, drop it to avoid spurious reordering.
• if the index has length(idx) == 0, retain it for all columns until the first non-existing (exclusive)

[codecov-io]

Codecov Report

Merging #2405 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2405      +/-   ##
=========================================
+ Coverage   91.29%   91.3%   +<.01%     
=========================================
  Files          62      62              
  Lines       12025   12027       +2     
=========================================
+ Hits        10978   10981       +3     
+ Misses       1047    1046       -1

Impacted Files	Coverage Δ
R/data.table.R	97.45% <100%> (ø)	⬆️
src/rbindlist.c	88.93% <0%> (+0.19%)	⬆️

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