This is useful for copying the entire content of the selected file as it was at
the selected commit.
We only add it to the commit files panel; it is not needed in the files panel,
because there you can simply press "e" to edit the file.
This is very similar to the same menu in the Files panel, except that it works
on whatever diff is currently shown in the main view, including range diffs
either in diffing mode (shift-W), or from a range selection of commits.
