add repohooks

By default, no hooks are enabled, so this shouldn't impact people.

Bug: 28295071
Change-Id: Ia74f03ea89f7b39faa61f26d7846f5e3cbb3f128
Test: `repo upload` in a repo didn't blow up
1 file changed
tree: f97702120119200efa820bea00cbac09372f9e12
  1. default.xml