Fix a process bug when running show commands on Windows

[damienstanton]

With the released version, all gitu commands for showing, editing etc. using the EDITOR env var and others will fail because we try to launch the subprocess command directly.

On Windows, this doesn't work; Commands need to be prepended with cmd /C to be executed in sub-shells.

[damienstanton]

Note that there are some tests that this currently breaks, so I'll fix these before calling it ready to merge.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.73%. Comparing base (c772624) to head (ead232d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #330      +/-   ##
==========================================
+ Coverage   87.71%   87.73%   +0.01%     
==========================================
  Files          63       63              
  Lines        5627     5636       +9     
==========================================
+ Hits         4936     4945       +9     
  Misses        691      691              

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