Additional removal of Commit concept

[rjkroege]

Now that newtype text buffers don't require Commit, further cleanup the code around this.

[codecov[bot]]

Codecov Report

Merging #458 (5ae6908) into master (6801519) will decrease coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
- Coverage   58.02%   57.98%   -0.04%     
==========================================
  Files          54       54              
  Lines       10683    10676       -7     
==========================================
- Hits         6199     6191       -8     
- Misses       4049     4050       +1     
  Partials      435      435              

Impacted Files	Coverage Δ
file/buffer_adapter.go	90.90% <ø> (-0.17%)	:arrow_down:
file/observable_editable_buffer.go	71.28% <ø> (-1.53%)	:arrow_down:
text.go	48.44% <100.00%> (-0.01%)	:arrow_down:

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