sync: Added -g as a short option for --no-content

sync: Added -g as a short option for --no-content

I anticipate that if sync is transitioned to syncing content by default,
people will want a short option. And in repositories where
annex.synccontent = true, they already would. And pull and push sync
content by default, so a short option is useful with them too.

Mnemonic: -g makes only git data be synced
Also, -a makes only annex data be synced.

Would have preferred -c, which would complement -C, but it
was already taken to set git configs.

Sponsored-by: Noam Kremen on Patreon