git-commit.txt: better description what it does

The description of git-commit jumps right into the commit content, which
is important, but it fails to mention how the commit is "added" to the
repository. Update the first paragraph saying a bit more about branch
update to fill this gap.

While at there, add a couple linkgit references when the command is
first mentioned.

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

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