magit-revision-refresh-buffer: Avoid file refname

When both a file and revision have the same name, 'git show' fails:

    fatal: ambiguous argument 'foo': both revision and filename
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'

As a result, `magit-revision-refresh-buffer' showed an empty buffer.

Use '--' to specify that this argument is a revision.
Merge pull request #1558 from kyleam/n/fix-rev-ambig

magit-revision-refresh-buffer: Avoid file refname