expand repoid to 16 chars and avoid querying git for it twice

expand repoid to 16 chars and avoid querying git for it twice

i.e., just reuse GITVERSION, and FWIW, the current ID length git
auto chooses is 9 chars, suggesting that 8 really was getting to
small. With 16 we got now 64 bits, that's plenty for the next few
years.

Document the format also (roughly) in the API schema

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>