Add a normalize() method to dirstate
This method returns the normalised form of a path. This is
- the form in the dirstate, if available, or
- the form on disk, if available, or
- the form passed on the command line
normalize() is called on the type-'f' result of statwalk.
This fixes issues 910 and 1092
% should fail
abort: path contains illegal component: .hg/00changelog.i
adding a/a
% should fail
abort: path 'b/b' traverses symbolic link 'b'
% should succeed
% should still fail - maybe
abort: path 'b/b' traverses symbolic link 'b'