Notice when git apply --check --cached returns an error.

This is good for not letting you enter commit mode if you have
a patch that doesn't pass "git apply" for whatever reason, such
as introduction of trailing whitespace.

gitsum.el

@@ -93,21 +93,22 @@ A numeric argument serves as a repeat count."
 (defun gitsum-commit ()
   "Commit the patch as-is, asking for a commit message."
   (interactive)
-  (shell-command-on-region (point-min) (point-max) "git apply --check --cached")
-  (let ((buffer (get-buffer-create "*gitsum-commit*"))
-        (dir default-directory))
-    (shell-command-on-region (point-min) (point-max) "(cat; git diff --cached) | git apply --stat" buffer)
-    (with-current-buffer buffer
-      (setq default-directory dir)
-      (goto-char (point-min))
-      (insert "\n")
-      (while (re-search-forward "^" nil t)
-        (replace-match "# " nil nil))
-      (forward-line 0)
-      (forward-char -1)
-      (delete-region (point) (point-max))
-      (goto-char (point-min)))
-    (log-edit 'gitsum-do-commit nil nil buffer)))
+  (when (zerop (shell-command-on-region (point-min) (point-max)
+                                        "git apply --check --cached"))
+    (let ((buffer (get-buffer-create "*gitsum-commit*"))
+          (dir default-directory))
+      (shell-command-on-region (point-min) (point-max) "(cat; git diff --cached) | git apply --stat" buffer)
+      (with-current-buffer buffer
+        (setq default-directory dir)
+        (goto-char (point-min))
+        (insert "\n")
+        (while (re-search-forward "^" nil t)
+          (replace-match "# " nil nil))
+        (forward-line 0)
+        (forward-char -1)
+        (delete-region (point) (point-max))
+        (goto-char (point-min)))
+      (log-edit 'gitsum-do-commit nil nil buffer))))
 
 (defun gitsum-amend ()
   "Amend the last commit."