Improve arc's handling of dirty submodules in Git

Summary:
Fixes T9455. Depends on D14136. When you have a dirty submodule:

$ nano submodule/file.c # save changes

...we currently ask you to make a commit when you run arc diff, which is meaningless and misleading.

Instead, prompt the user separately.

This behavior isn't perfect but I think it's about the best we can do within reason.

Test Plan:

• Ran arc diff in a working copy with uncommitted submodule changes only, got new prompt.
• Ran arc diff in a working copy with submodule base commit changes only, got old (correct) prompt.
• Ran arc diff in a working copy with both, got only old prompt (which is incomplete, but reasonable/meaningful).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9455

Differential Revision: https://secure.phabricator.com/D14137