Add a magit-current-range variable for diff and log buffers.

magit.el

@@ -565,6 +565,16 @@ return nil."
 
 ;;; Revisions and ranges
 
+(defvar magit-current-range nil
+  "The range described by the current buffer.
+This is only non-nil in diff and log buffers.
+
+This has three possible (non-nil) forms.  If it's a string REF or
+a singleton list (REF), then the range is from REF to the current
+working directory state (or HEAD in a log buffer).  If it's a
+pair (START . END), then the range is START..END.")
+(make-variable-buffer-local 'magit-current-range)
+
 (defun magit-list-interesting-refs ()
   (let ((refs ()))
     (dolist (line (magit-git-lines "show-ref"))
@@ -3577,6 +3587,7 @@ With a non numeric prefix ARG, show all entries"
 (defun magit-refresh-log-buffer (range style args)
   (magit-configure-have-graph)
   (magit-configure-have-decorate)
+  (setq magit-current-range range)
   (magit-create-log-buffer-sections
     (apply #'magit-git-section nil
 	   (magit-rev-range-describe range "Commits")
@@ -3688,6 +3699,7 @@ level commits."
 ;;; Diffing
 
 (defun magit-refresh-diff-buffer (range args)
+  (setq magit-current-range range)
   (magit-create-buffer-sections
     (magit-git-section 'diffbuf
 		       (magit-rev-range-describe range "Changes")