Add rev_save as shim for core's save

[kyleam]

datalad/datalad#3408 replaces core's save with rev_save. Add back a
compatibility alias, like we already do for other commands.

[codecov]

Codecov Report

Merging #127 into master will increase coverage by 0.36%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage    77.5%   77.86%   +0.36%     
==========================================
  Files          10       10              
  Lines         120      131      +11     
==========================================
+ Hits           93      102       +9     
- Misses         27       29       +2

Impacted Files	Coverage Δ
datalad_revolution/__init__.py	100% <ø> (ø)	⬆️
datalad_revolution/revsave.py	87.5% <83.33%> (-12.5%)	⬇️

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