Save changes in DataLad if only the statefile was modified

[jwodder]

Also, don't migrate the statefile if there were no changes.

Fixes #119.

[codecov-commenter]

Codecov Report

Merging #121 (1071c62) into master (af8fe5f) will decrease coverage by 1.48%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   72.00%   70.51%   -1.49%     
==========================================
  Files          10       10              
  Lines         918      926       +8     
  Branches      135      138       +3     
==========================================
- Hits          661      653       -8     
- Misses        202      220      +18     
+ Partials       55       53       -2     

Impacted Files	Coverage Δ
src/tinuous/__main__.py	51.74% <33.33%> (-1.49%)	⬇️
src/tinuous/state.py	100.00% <100.00%> (ø)
src/tinuous/github.py	47.36% <0.00%> (-4.31%)	⬇️
src/tinuous/travis.py	63.63% <0.00%> (-2.73%)	⬇️
src/tinuous/appveyor.py	76.19% <0.00%> (-1.20%)	⬇️
src/tinuous/base.py	88.18% <0.00%> (+0.78%)	⬆️

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[yarikoptic]

Thank you, looks great!