Include file names in challenge_uncommited_changes()

[olivroy]

Proof of concept to make the message shorter, and more focused on current situation.

Adding a which argument that conflicts with msg.

Rephrase message to be shorter.

Add a link to the uncommited files with {.file. format_inline() will shorten automatically as needed.

I think it is worth adding files in the message, since it can facilitate the choice.

Let me know what you think!

This PR:

Before

1. If I know which files are not commited, I spend more time figuring out if I should press 1, 2, or 3 for yes or no is than actually looking if I'm okay with continuing.
2. If I don't know what files, I have to go look elsewhere to figure out which files are not commited, and then go to do step 1.

---

Yields the following message (instead of the correctly formatted with ! before.

    There are uncommitted changes and we're about to create and push to a new \
    GitHub repo 

[jennybc]

I think this is a good pattern to reuse in this scenario:

> usethis::use_dev_version(push = TRUE)
✔ Adding "3.0.0.9000" to Version.
✔ Adding new heading to NEWS.md.
ℹ There are 2 uncommitted files:
• DESCRIPTION
• NEWS.md
! Is it ok to commit and push them?
...

This is ultimately coming from git_ask_commit(). I suspect there might be some refactoring that would help to share this code in more situations.