BF: gh-3997, never use the worktree as reference when diffing against a commit

[mih]

Story in gh-3997

[mih]

This must go into 0.12, hence quick feedback is appreciated.

[codecov]

Codecov Report

Merging #3999 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3999      +/-   ##
==========================================
+ Coverage   89.68%   89.69%   +<.01%     
==========================================
  Files         271      271              
  Lines       36382    36394      +12     
==========================================
+ Hits        32631    32643      +12     
  Misses       3751     3751

Impacted Files	Coverage Δ
datalad/support/gitrepo.py	89.47% <ø> (ø)	⬆️
datalad/core/local/tests/test_diff.py	99.48% <100%> (+0.03%)	⬆️
datalad/log.py	82.69% <0%> (-7.22%)	⬇️
datalad/tests/test_log.py	98.03% <0%> (-0.99%)	⬇️
datalad/downloaders/tests/test_http.py	60.58% <0%> (+2.18%)	⬆️
datalad/downloaders/http.py	74.9% <0%> (+2.78%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 90caee8...7d9f19d. Read the comment docs.