Adding mpm to the nosy list and attaching a simpler test case.

There are two problems here:

- we should require hg merge --force when there are deleted files

- we should fix handling of deleted files.

For the first one we just have to fix the test for modified working dir.
Right now it looks like "if wc.files():" , but workingctx.files() doesn't
include deleted files.  I'm not sure if it's better to include them there,
or to change the test to 'if wc.files() or wc.deleted():'.

I'll attach a possible patch for the second problem in a bit.