satellite: use double-dash to disambiguate refspec and pathspec

When a file or directory is named the same as either the source or
target branch then git will fail to diff because of the ambiguity.  e.g.
if you touch a file called "master" you will see this output:

	$ git diff HEAD master
	fatal: ambiguous argument 'master': both revision and filename
	Use '--' to separate paths from revisions, like this:
	'git <command> [<revision>...] -- [<file>...]'

The solution is to use the double-dash to separate paths from revisions,
as git suggests in its error message.

Related-to: #4944

Signed-off-by: David Aguilar <davvid@gmail.com>