Description of problem:

meld <file> is supposed to start a version control comparison, and it indeed does so in a (say) Subversion work directory.  However, this is broken for Mercurial when <file> is not at the same level as the special .hg directory.

Version-Release number of selected component (if applicable):

I've tried meld 1.3 to 1.5.1 (rawhide).

How reproducible:

Always.

Steps to Reproduce:
1. Create a Mercurial repository (hg init w)
2. Add a file and modify it (mkdir w/d; cd w/d; echo empty > f; hg add f; hg ci -m test; echo more >> f)
3. Launch meld with <file> (meld f)
  
Actual results:

meld GUI with one panel, no diff.

Expected results:

meld GUI with two panels, with diff.