gitweb: In incremental builds, regenerate each new parent directory

If the new file is created in a path with multiple new parent
directories, then we need to regenerate each parent directory.
Further, we need to rebuild the index page of the directory
containing the first new directory entry.

I ran into this while upgrading the depp HTML for the quebex repo
from 79e4c52693d71cffec7d45a1eed683c6c7ab93ee to commit
e3bfecd2465361a61e51ca54304e19775c6a75e5.

This is a fixup for commit 8860544.

Update git2go dependency to support libgit2 1.8

Fixes #1

gitweb: Don't panic