ikiwiki/doc/bugs/conflicts.mdwn

The `conflicts` testcase has 4 failing test cases. The underlaying problem 
is that there are multiple possible source files that can create the same
destination files.

1. `foo.mdwn` is in srcdir, rendered to destdir. Then
   it is removed, and `foo` is added, which will be rendered
   raw to destdir. Since the `foo/` directory still exists,
   it fails.
1. `foo` is added to srcdir, rendered raw to destdir.
   Then it is removed from srcdir, and `foo.mdwn` is added.
   The `foo` file is still present in the destdir, and mkdir
   of the directory `foo/` fails.
1. `foo.mdwn` renders to `foo/index.html`. Then `foo/index.html`
   is added to the srcdir, using rawhtml. It renders to the same 
   thing.
1. `foo/index.html` in srcdir is rendered to same thing in destdir
   using rawhtml. Then `foo.mdwn` is added; renders same thing.

Note that another case, that of page `foo.mdwn` and page `foo.txt`, that
both render to `foo/index.html`, used to cause problems, but no longer
crashes ikiwiki. It now only complains in this situation, and which
file "wins" is undefined. The fix for this relied on both pages being
named `foo`; but in the above cases, the source files have different
pagenames.

One approach: Beef up checking in `will_render` to detect when the same
destination file is rendered by multiple pages. Or when one page renders
a file that is a parent directory of the rendered file of another page.
It could warn, rather than erroring. The last page rendered would "win";
generating the destdir file.

[[done]]