Skip VPP build for merge jobs with Gerrit ID != git HEAD.

Skip VPP build for merge jobs with Gerrit ID != git HEAD.

- Since merge jobs are serialized after the patches have
  been merged into git, all merge jobs which do not have
  the same Gerrit ID as git HEAD are redundant.  Skip
  them to save useless cycles and to reduce the size of
  the build queues when multiple patches are merged in a
  short amount of time.

- This also fixes the issue with packagecloud packages
  containing different contents then the associated
  Gerrit ID when built & pushed in this state.

Change-Id: I822492bd1d523576373bb268258c181a9c5edb42
Signed-off-by: Dave Wallace <[email protected]>