Modified the revision selection while comparing po-files$

Modified the revision selection while comparing po-files$

If revision-parameter is set to e.g. '-3', now we search for the previous
3-rd change of the relevant po-file instead of the previous 3-rd commit.

We now also pass options through the environment, though it is not needed for git.
This way we overcome the deficiency of "hg" and "svn", which do not allow
to pass self-defined options to underlying diff-command.