[#31254] Change Git commit command in "Existing folder" to git commit -m "First commit"

What does this MR do?

From https://gitlab.com/gitlab-org/gitlab-ce/issues/31254 We show "Command line instructions" when the repository of a newly created project is empty. In the "Existing folder section, one of the commands is git commit. If these commands are copied and pasted in the shell, the last command ends up in the editor. In some cases, the editor doesn't start and git commit fails with an empty commit message.

This MR changes the git commit instruction with a -m flag to allow it to be copy-paste

Why was this MR needed?

Resolve https://gitlab.com/gitlab-org/gitlab-ce/issues/31254

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/31254

Closes #31254 (closed)