script to verify Gerrit synchronization

We sometime have extensions improperly configured in either Gerrit or
the submodule file.  The check-sync.sh script would pull all extensions
from Gerrit and verify that there is no commits between their HEAD and
origin/master.

Example output with mediawiki/extensions.git at d0680b9 :

ERROR!
AutoCreateCategoryPages is lagging behind.
ERROR! Bootstrap is lagging behind.
ERROR! Campaigns is lagging behind.
ERROR! CirrusSearch is lagging behind.
ERROR! CommunityTwitter is lagging behind.
ERROR! CoreEvents is lagging behind.
ERROR! EImage is lagging behind.
ERROR! JsonData is lagging behind.
ERROR! Less is lagging behind.
ERROR! OpenStreetMapSlippyMap is lagging behind.
ERROR! PerPageLicense is lagging behind.
ERROR! QuickResponse is lagging behind.
ERROR! TimelineTable is lagging behind.
ERROR! VisualEditor is lagging behind.
ERROR! WikibaseDataModel is lagging behind.
ERROR! WikibaseQueryEngine is lagging behind.

bug: 49906
Change-Id: I1d294afb92c08f1268934eed84f0e53ab8f8dbf1