stgit.el: Add support for showing which files are affected by a patch

stgit.el: Add support for showing which files are affected by a patch

One can "expand" a patch by pressing RET, which shows the files that
patch modifies. On a line with a modified file, one can:
  RET  find file
  o    find file in other window
  =    show diff in that file

\C-n and \C-p can be used to move between patches when they have been
expanded.

Signed-off-by: Gustav Hållberg <gustav@virtutech.com>
Signed-off-by: Karl Hasselström <kha@treskal.com>