magit-revision-refresh-buffer: mention file filter in header line

Documentation/RelNotes/2.5.1.txt

@@ -42,6 +42,9 @@ Fixes since v2.5.0
   `magit-pre-call-git-hook' and logging its git command to the
   magit-process buffer.
 
+* When a revision buffer was limited to a set of files, its header
+  line did not say so.
+
 * On `windows-nt' systems, having `mount' in `exec-path' caused Magit
   to assume that git is from Cygwin.
 

lisp/magit-diff.el

@@ -1508,9 +1508,15 @@ Staging and applying changes is documented in info node
   :group 'magit-revision
   (hack-dir-local-variables-non-file-buffer))
 
-(defun magit-revision-refresh-buffer (rev __const _args _files)
+(defun magit-revision-refresh-buffer (rev __const _args files)
   (setq header-line-format
-        (propertize (format " %s %s" (capitalize (magit-object-type rev)) rev)
+        (propertize (concat " " (capitalize (magit-object-type rev))
+                            " " rev
+                            (pcase (length files)
+                              (0)
+                              (1 (concat " in file " (car files)))
+                              (_ (concat " in files "
+                                         (mapconcat #'identity files ", ")))))
                     'face 'magit-header-line))
   (magit-insert-section (commitbuf)
     (run-hook-with-args 'magit-revision-sections-hook rev)))