Make phpcs-changed work in Windows without WSL

[sirbrillig]

• Move all git workflow commands (eg: calls to git or phpcs) into methods on ShellOperator so that they can be implemented separately for each OS.
• Move all svn workflow commands (eg: calls to svn or phpcs) into methods on ShellOperator so that they can be implemented separately for each OS.
• Remove executeCommand on ShellOperator.
• Create a ShellOperator implementation for Windows.

[sirbrillig]

Just the git workflow commands that need to be converted.

• Does git file exist: Move git file existence check to ShellOperator #74
• Get git root directory, phpcs output, git add, and git hash: Move hash, phpcs, and root git commands to ShellOperator #75
• Get git merge base: Move git merge base to shell #77
• Get git unified diff: Move getting unified git diff to shell #76

I'll still need to see what's required for svn mode.

[shivapoudel]

@sirbrillig Is this issue resolved or is it in progress?

[sirbrillig]

This issue is in-progress. See the checklist in the issue description.

[sirbrillig]

All the unix-specific code should now be moved to ShellOperator, which means we should be able to try implementing a Windows version now.