gitweb: Fix bug in evaluate_path_info

There was bug in parsing "project/:/file" and "project/:/" path_info
URLs, with implicit HEAD as 'hash_base'.

For such URLs the refname is empty, and before this fix regexp for
parsing path_info fragment assumed that it is always non-empty.

Refname cannot contain ':', as per 'git check-ref-format'.

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