[update-checkout] add the 'status' subcommand

[charles-zablit]

This patch adds the status command to update-checkout to quickly check the status of all the git repositories.

The status command prints the list of repositories with active changes (uncommitted, untracked and/or staged). This subcommand should help users determine if it's safe to run update-checkout --clone --reset-to-remote.

Examples

When some repositories have active changes:

> ./swift/utils/update-checkout status
The following repositories have active changes:
  - [swift] 1 active change
  - [llvm-project] 2 active changes

When no repository has active changes:

> ./swift/utils/update-checkout status
No repositories have active changes.

rdar://165437920

[charles-zablit]

@swift-ci please smoke test