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>

git-cvsserver.perl

@@ -2543,8 +2543,15 @@ sub update
                     if ($parent eq $lastpicked) {
                         next;
                     }
-                    my $base = safe_pipe_capture('git-merge-base',
+		    my $base = eval {
+			    safe_pipe_capture('git-merge-base',
 						 $lastpicked, $parent);
+		    };
+		    # The two branches may not be related at all,
+		    # in which case merge base simply fails to find
+		    # any, but that's Ok.
+		    next if ($@);
+
                     chomp $base;
                     if ($base) {
                         my @merged;