Allow unmarking the marked commit.

Once, I hit "." instead of "r ." by accident and have found no way to
reset this mark. This just seems not to be right.

magit.el

@@ -2359,11 +2359,15 @@ Prefix arg means justify as well."
     ((commit)
      (magit-show-commit info #'scroll-down))))
 
-(defun magit-mark-item ()
-  (interactive)
-  (magit-section-action (item info "mark")
-    ((commit)
-     (magit-set-marked-commit info))))
+(defun magit-mark-item (&optional unmark)
+  (interactive "P")
+  (if unmark
+      (magit-set-marked-commit nil)
+    (magit-section-action (item info "mark")
+      ((commit)
+       (magit-set-marked-commit (if (eq magit-marked-commit info)
+				    nil
+				  info))))))
 
 (defun magit-describe-item ()
   (interactive)

magit.texi

@@ -253,10 +253,13 @@ kill ring.
 Typing @kbd{=} will show the differences from the current commit to
 the @dfn{marked} commit.
 
-You can mark the current commit by typing @kbd{.}.  Some commands,
-such as @kbd{=}, will use the current commit and the marked commit as
-implicit arguments.  Other commands will offer the marked commit as a
-default when prompting for their arguments.
+You can mark the current commit by typing @kbd{.}.  When the current
+commit is already marked, typing @kbd{.} will unmark it.  To unmark
+the marked commit no matter where point it, use @kbd{C-u .}.
+
+Some commands, such as @kbd{=}, will use the current commit and the
+marked commit as implicit arguments.  Other commands will offer the
+marked commit as a default when prompting for their arguments.
 
 @node Reflogs
 @chapter Reflogs