completion: improve bash completion for git-add

completion: improve bash completion for git-add

Command completion for git-add did not recognize some long-options.
This commits adds completion for all long-options that are mentioned in
the man-page synopsis. In addition, if the user specified `--update` or
`-u`, path completion will only suggest modified tracked files.

Signed-off-by: Cornelius Weig <cornelius.weig@tngtech.com>
Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>