gotwebd: fix branches modification time with show_repo_age off
If show_repo_age is set to off, got_get_repo_age() returns without
setting the timestamp. got_get_repo_age() is also used by the
branch listing to show how old they are however, and we end up
rendering the stack garbage.
Instead, respect `show_repo_age off' one layer above, and always
return a timestamp in got_get_repo_age(). While here, initialize
the timestamp to zero so it's set also in case of failures.
Issue reported on IRC by xs, thanks!