rewrite add/changed/modified functions

[lukaszb]

In terms of current a/c/m attributes and usage of files() after some thinking i believe that usage of
localrepo.status() would be much better.

For example
hg st --change REV gives You modified, added, removed, deleted files for that REV, thus making merge changes same as in diff.

And it's much faster than current try except solution.

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Reference: https://bitbucket.org/marcinkuzminski/vcs/issues/35/