completion: teach remote subcommands to complete options

Git-remote needs to complete remote names, its subcommands, and options
thereof. In addition to the existing subcommand and remote name
completion, do also complete the options

- add: --track --master --fetch --tags --no-tags --mirror=
- set-url: --push --add --delete
- get-url: --push --all
- prune: --dry-run

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>