BF: update -- do not save if no --merge, use save instead of Dataset(refds_path).add

[yarikoptic]

refds_path could be empty if dataset is not specified but path (to subdataset) is provided

Note that it would change existing behavior -- apparently we could have been saving some prior changes if there were some before running update. I would consider that also to be a bug fix

Fixes #3097
ref: templateflow/templateflow#5

TODOs (after merge)

• merge/adjust for master way of things

[codecov]

Codecov Report

Merging #3996 into 0.11.x will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           0.11.x    #3996      +/-   ##
==========================================
+ Coverage   89.92%   89.96%   +0.04%     
==========================================
  Files         253      253              
  Lines       34078    34085       +7     
==========================================
+ Hits        30643    30666      +23     
+ Misses       3435     3419      -16

Impacted Files	Coverage Δ
datalad/distribution/tests/test_update.py	100% <100%> (ø)	⬆️
datalad/distribution/update.py	98.14% <100%> (+0.01%)	⬆️
datalad/downloaders/tests/test_http.py	91.15% <0%> (+2.21%)	⬆️
datalad/downloaders/http.py	87.3% <0%> (+2.77%)	⬆️

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[bpoldrack]

LGTM