git: Hard wrap in editor (#26507)

This adds the ability for the editor to implement hard wrap (similar to
"textwidth" in vim).

If you are typing and your line extends beyond the limit, a newline is
inserted before the most recent space on the line. If you are otherwise
editing the line, pasting, etc. then you will need to manually rewrap.

Release Notes:

- git: Commit messages are now wrapped "as you type" to 72 characters.