cvsserver: Fix for histories with multiple roots

cvsserver: Fix for histories with multiple roots

Git histories may have multiple roots, which can cause
git merge-base to fail and this caused git cvsserver to die.

This commit teaches git cvsserver to handle a failing git
merge-base gracefully, and modifies the test case to verify this.
All the test cases now use a history with two roots.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
 git-cvsserver.perl              |    9 ++++++++-
 t/t9400-git-cvsserver-server.sh |   10 +++++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)
Signed-off-by: Junio C Hamano <gitster@pobox.com>