git chain push should set upstream automatically if one does not exist

[dashed]

git chain push should set upstream (remote) automatically if one does not exist. This is more smarter than bailing out naively:

git-chain/src/main.rs

Lines 340 to 347 in f51494c

	if e.code() == ErrorCode::NotFound {
	println!(
	"🛑 Cannot push. Branch has no upstream: {}",
	self.branch_name.bold()
	);
	// do nothing
	return Ok(false);
	}

Do not do this for git chain push --force since it's a dangerous operation.

Bonus: if git chain push --force fails, then suggest that the user run git chain push if upstream (remote) branches are detected to not be set.

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Starting points:

• https://docs.rs/git2/latest/git2/struct.Repository.html#method.remotes
• https://docs.rs/git2/0.13.25/git2/struct.Branch.html#method.set_upstream
• https://docs.rs/git2/0.13.25/git2/struct.Branch.html#method.set_upstream