fix test suite's dependency on time zone

I had not remembered that the 'tc' subcommand in tsh adds *text* that
contains the current time, so commit SHAs were changing.

Thanks to milki for catching this, and in fact being the only person who
ever appears to have attempted to run the test suite at all!

src/lib/Gitolite/Test/Tsh.pm

@@ -583,7 +583,7 @@ sub dummy_commits {
             test_tick();
             next;
         }
-        my $ts = ( $tick ? localtime($tick) : localtime() );
+        my $ts = ( $tick ? gmtime($tick+19800) : gmtime() );
         _sh("echo $f at $ts >> $f && git add $f && git commit -m '$f at $ts'");
     }
 }