help: replace underlining "help -a" headers using hyphens with a blank line

help: replace underlining "help -a" headers using hyphens with a blank line

We used to underline a header text, like this:

    This is a header
    ----------------
    content...

But calculating text length so that the dashes align with the text
could get complicated because the text could be in any charset in
translated Git.

There is no point to use this pseudo underline; simply a blank
line would do and it even makes it easier to read:

    This is a header

    content...

While at it, give translators more context to translate, e.g.
e.g.  "git commands available..." instead of "%s available..."

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>