autoindex test: check what happens to files in transient/normal underlays
autoindex: remove unnecessary special case for transient underlay

It's not clear that the transient underlay should be excluded from
indexing; see [[bugs/transient autocreated tagbase is not transient
autoindexed]].

In any case, the code that checks what directories might need indexes
specifically checks for the srcdir anyway, so the only effect this extra
check can have is negative (it could fail to notice files in the
transient underlay and attempt to recreate them unnecessarily).
Add a regression test for autoindex_commit => 1
Allow creation of transient index pages for directories outside srcdir

After this change autoindex creates index pages also for empty directories
included in underlays, but only if it isn't going to commit them to the
srcdir ($config{autoindex_commit} = 0).

Inspired by a patch from Tuomas Jormola.

Bug-Debian: http://bugs.debian.org/611068
Merge branch 'ready/autoindex-more-often'
Merge branch 'master' of ssh://ikiwiki.branchable.com
changelog, close bug