Translate git-add.txt based on my past work.

Signed-off-by: Katsuyuki TATEISHI <kt@wheel.jp>
Merge branch 'ja_JP.work'

* ja_JP.work:
  Translate git-add.txt based on my past work.
Delete original part from git-add.txt

Signed-off-by: Katsuyuki TATEISHI <kt@wheel.jp>