Don't convert to absolute path

Previously we converted any path given to nvr to an absolute one. But then `nvr
http://example.com/file` wouldn't trigger netrw's BufReadCmd event and thus
download the file, because it would see an absolute path like
`~/http://example.com/file` and the netrw autocmd only matches on `http://*`.

Fixes #86

nvr/nvr.py

@@ -85,7 +85,7 @@ class Nvr():
         self.server.command('silent 1delete _ | set nomodified')
 
     def fnameescaped_command(self, cmd, fname):
-        path = self.server.funcs.fnameescape(os.path.abspath(fname))
+        path = self.server.funcs.fnameescape(fname)
         self.server.command('{} {}'.format(cmd, path))
 
     def diffthis(self):