Rather than relying only on the name of the group, use the full DN
of the group to match from LDAP to Gerrit's database. This prevents
identically named groups under different trees from colliding with
each other and producing ambiguous results.
Change-Id: I389a49f60f7bf9fcb6df17d0ca5547a1fce8f9a2
Signed-off-by: Shawn O. Pearce <sop@google.com>