`GitModified` and `GitStaged` filters blindly expect that the `git` command is available

[fredden]

Is your feature request related to a problem?

When the GitModified or GitStaged filters are used, the code blindly expects that the git command-line tool is available. This results in unexpected behaviour if this assumption is not true.

Describe the solution you'd like

It would be better for end users if a suitable error message (and exit code) is presented describing the problem.

Additional context (optional)

This suggestion was initially raised in #1105 (review)

• I have read the Contribution Guidelines and this is not a support question.
• I intend to create a pull request to implement this feature.

[jrfnl]

I wonder whether it would even make sense to do this validation as early as in the Config class ?