make units explicit on implicit difftime by group

[MichaelChirico]

Closes #3694 and #761 (and there are several duplicates already closed as well)

If we think the overhead of the !inherits(e2, 'POSIXt') --> base::-.POSIXt case is too high, we could copy-paste the rest of the -.POSIXt body into our mask...

[codecov]

Codecov Report

Merging #3837 into master will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #3837      +/-   ##
==========================================
- Coverage   99.42%   99.41%   -0.01%     
==========================================
  Files          71       71              
  Lines       13408    13415       +7     
==========================================
+ Hits        13331    13337       +6     
- Misses         77       78       +1

Impacted Files	Coverage Δ
R/data.table.R	99.94% <85.71%> (-0.06%)	⬇️

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[codecov]

Codecov Report

Merging #3837 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3837      +/-   ##
==========================================
+ Coverage   99.42%   99.42%   +<.01%     
==========================================
  Files          71       71              
  Lines       13416    13423       +7     
==========================================
+ Hits        13339    13346       +7     
  Misses         77       77

Impacted Files	Coverage Δ
R/data.table.R	100% <100%> (ø)	⬆️

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