Give git process clues in SCM input placeholder

[gjsjohnmurray]

This PR uses the SCM input placeholder to help a git novice know what to do.

When no changes exist:

When changes or untracked files exist but nothing has been staged yet:

After staging something the usual placeholder appears:

[lszomoru]

@gjsjohnmurray, thank you very much for your contribution. Rather than going down the path that is being proposed in this pull request, I would like to factor in this feedback in the work that is currently being tracked in #135992. I am hoping that this is something that we could probably take a look at next milestone.

//cc @misolori

[miguelsolorio]

Appreciate the PR! Is there an original issue for this? Would be great to have a place to discuss before the PR.

[gjsjohnmurray]

@misolori I confess I went straight to creating this PR. Pretty sure it arose from having responded to an issue in which a user was having a problem with the SCM view and seemed not to have grasped the sequence of steps. But I can't find that issue any more.

I created a branch to try out the idea on, then it seemed simpler to submit it as a PR than open a discussion issue first. No problem with me if you want to close it unmerged.

[gjsjohnmurray]

Found the issue which prompted it - #142335

[lszomoru]

@gjsjohnmurray, thanks again for your contribution. Today's Insiders release (2022-06-27) contains a Commit new action button which hopefully help users with committing their changes. Please give it a try and let me know what you think. I would like to further improve that experience in favour of using the placeholder text in the SCM Input. If you do not mind I will go ahead and close this PR.