Hi. This bug has been there since at least emacs23 it seems like. To
reproduce:

1. emacs -Q

2. Open any patch file that has hunks from more than one file in it. For
instance, the attached patch file works, but it really doesn't matter

3. Narrow to just the second file's diffs. For instance: M-N M-N C-SPC
C-> C-x n n SPC (last SPC to confirm the narrowing)

4. Try to see the source of the hunk. Move the point to the guts of the
hunk, and M-enter. Emacs asks for the directory where the sources live.
In the process, it changes the narrowing to the wrong hunk. This is
wrong. The narrowing shouldn't change.

5. If we cancel with C-g, the wrong narrowing persists