Make "s" in git-status acknowledge the currently marked files

gitsum.el

@@ -38,7 +38,7 @@ This mode is meant to be activated by `M-x gitsum' or pressing `s' in git-status
 
 ;; When git.el is loaded, hack into keymap.
 (when (boundp 'git-status-mode-map)
-  (define-key git-status-mode-map "s" 'gitsum))
+  (define-key git-status-mode-map "s" 'gitsum-switch-from-git-status))
 
 ;; Undo doesn't work in read-only buffers else.
 (defun gitsum-undo ()
@@ -50,15 +50,15 @@ A numeric argument serves as a repeat count."
   (let ((inhibit-read-only t))
     (undo)))
 
-(defun gitsum-refresh ()
+(defun gitsum-refresh (&optional arguments)
   "Regenerate the patch based on the current state of the index."
   (interactive)
   (let ((inhibit-read-only t))
     (erase-buffer)
     (insert "# Directory:  " default-directory "\n")
     (insert "# Use n and p to navigate and k to kill a hunk.  u is undo, g will refresh.\n")
     (insert "# Edit the patch as you please and press 'c' to commit.\n\n")
-    (let ((diff (shell-command-to-string "git diff")))
+    (let ((diff (shell-command-to-string (concat "git diff " arguments))))
       (if (zerop (length diff))
           (insert "## No changes. ##")
         (insert diff)
@@ -156,6 +156,16 @@ A numeric argument serves as a repeat count."
   (interactive)
   (git-status default-directory))
 
+(defun gitsum-switch-from-git-status ()
+  "Switch to gitsum, resticting diff to marked files if any."
+  (interactive)
+  (let ((marked (git-get-filenames
+                 (ewoc-collect git-status
+                               (lambda (info) (git-fileinfo->marked info))))))
+    (gitsum)
+    (when marked
+      (gitsum-refresh (mapconcat 'identity marked " ")))))
+
 (defun gitsum ()
   "Entry point into gitsum-diff-mode."
   (interactive)