This commit adds a new field to the group index to store the SHA1 of the
groups ref that the index document belongs to. With that, staleness can
be detected by comparing the value of the field to the current HEAD SHA1
on the groups ref. This mechanism is similar to what was used for
changes and accounts.
This commit also adds a staleness checker for groups as well as a test.
Change-Id: I113546ecd27079c03bbd1db728a22483db0a7ee2
24a7d37a0c