Make phpcs-changed work in Windows without WSL

[sirbrillig]

• [x] 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.
• [x] 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.
• [x] Remove executeCommand on ShellOperator.
• [ ] Create a ShellOperator implementation for Windows.

[sirbrillig]

Just the git workflow commands that need to be converted.

• [x] Does git file exist: https://github.com/sirbrillig/phpcs-changed/pull/74
• [x] Get git root directory, phpcs output, git add, and git hash: https://github.com/sirbrillig/phpcs-changed/pull/75
• [x] Get git merge base: https://github.com/sirbrillig/phpcs-changed/pull/77
• [x] Get git unified diff: https://github.com/sirbrillig/phpcs-changed/pull/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.