git: Use merge-base instead of log for testing if a commit is present

The current use of git log to check if a given revision is present can be
a little fragile.

For example if revision X was on branch A, and then later added to branch
B, the update checks would not notice this since they just check for X
being in the repository.

We also had some autobuilder corruption where an older packed-refs file
was copied over a new repository containing newer pack files. There
was no update to the refs file since the revision was present but
not accessible in any branch.

The correct fix is to check that the required revisions are present
on the specific branches. This patch does this using merge-base.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
serv.py: Give pr-server up to 5 seconds to commit data

The default value of 0.5 seconds before sending the pr-server a
SIGTERM is not enough to guarantee that sqlite has committed all
the pr data to the database. By polling the pid to see if it is
still running, this allows the pr-server process to shutdown
cleanly and finish the final pr data commit.

Signed-off-by: Konrad Scherer <Konrad.Scherer@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>