minor optimisation

minor optimisation

As soon as a change happens, we know we will need to rescan all
dependencies from the start, so bail out of the current scan partway to
avoid doing redundant work.

Only problem with this is that ikiwiki sometimes ends up printing out
dependencies that, while correct, are not obvious. Before:

building B, which depends on A
building C, which depends on A
building D, which depends on A

After:

building B, which depends on A
building C, which depends on B
building D, which depends on C