gitweb: Broken links in blame view

Hi!

I (or rather, the link checker) has discovered another broken link, this 
time in gitweb's blame view, which happens if you blame files and it 
tries to get parents before the initial (first) commit in the repository:

1. Unpack test.git.tar.gz (attached) into gitweb's project root.

2. Make sure $feature{'blame'}{'default'} = [1]; is on in
gitweb_config.perl.

3. Open the following page:
http://localhost/.../gitweb.cgi?p=test.git;a=blame;f=file;h=f8c58d2c88cbcf660f473f8f123ee32db0d0d2be;hb=HEAD

4. Click the gray "1" line number -- you get a "400 - Invalid hash base 
parameter" response (or '403' instead of 400 if you're not on next).

(I've also reproduced this with gitweb revisions from 7 months ago, so 
it's not related to recent changes.)

Also, probably related, if you open the above blame view by calling 
gitweb.cgi on the command line, you'll see a bogus error message on 
STDERR: "fatal: ambiguous argument 'f891e539f...^': unknown revision" 
Note that f891... is the earliest revision in the repository.

-- Lea