largefiles: extract and reuse 'standin' variable in overriderevert()

largefiles: override cmdutil.revert() instead of comands.revert() By overriding the cmdutil method we don't need to override both the function and the command. Also, we get access to the 'ctx' and 'parents' variables, which will soon prove useful. Rename the 'ctx' argument to overridematch() to 'mctx' rather than letting it shadow new 'ctx'.