Only guess default branch when a repo page is requested

Only guess default branch when a repo page is requested

There's no need to invoke guess_defbranch() for each repo during
scan-path, since repo.defbranch is only used when repo content is
being displayed.

Also, some users prefer to register their projects manually in cgitrc
but they got no benefit from the new repo.defbranch handling.

This patch tries to rectify these issues by only invoking guess_defbranch()
when needed, regardless of how the repo was registered.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>