gitweb: Fix actionless dispatch for non-existent objects

When gitweb URL does not provide action explicitly, e.g.

  http://git.example.org/repo.git/branch

dispatch() tries to guess action (view to be used) based on remaining
parameters.  Among others it is based on the type of requested object,
which gave problems when asking for non-existent branch or file (for
example misspelt name).

Now undefined $action from dispatch() should not result in problems.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>